(Some necessary overhead first.) If not familiar with white balance, then start by simply taking two pictures using intentionally wrong white balance. Take a picture outside in sunshine using Incandescent white balance (it will be very blue). Take another picture indoors in incandescent light using Daylight white balance (will be very orange). See? The lights are different colors, and the idea is to use proper white balance to match the light color, to get good color results in various lighting. But the problem is that there are many types and colors of lighting, which affect the color in our photographs. Usually the color in our photos won't be quite right, it could be better.
We may not realize the degree of this. Our human brain tries to fool us. Or rather, our brain amazingly knows to adjust our own white balance, and it can ignore these differences (in common situations). Humans recognize the color that subject things should be, and we often see what we expect to see. So when we come indoors, from out of cloudy 6000°K sunlight into incandescent 3000°K light, we don't realize that incandescent is orange. This (and more) is named the Land Effect, explained by Edwin H Land (of Polaroid fame). But cameras have no brain, and they do see and capture the color of real life, which we then can see is orange, which becomes a problem in our photos. There's a big difference, which photographers have to deal with. Auto WB does try to fix this, halfway well, but it has no clue about the right color either. We have ways to do it better ourselves.
So here and now, this subject of White Balance Correction is about when you set the camera right, and still get poor colors (color a little wrong is very common, not an exact system). But this is about how you can fix it and salvage a perfect picture. It's not difficult, but it is something we must tend to. Some users of course think their camera always gets color right. IMO, they have not realized yet that it's not. :) But once we start learning, and start seeing good color, correct color will become extremely important to us. White Balance is just a fact of life in Photography. It is a difficult problem, but we have ways to make it easy.
This animated histogram at right shows the concept of what White Balance actually does. Red and blue channels are shifted oppositely, here to align (balance) the RGB component peaks of a white card (which we know should be white) to remove a blue color cast. We can't see the photo here, but an image "too blue" is made less blue and more red, to become white. Because we have the known white card in the photo, we can see that in the histogram. White is a bright gray tone where neutral is defined as equal RGB components, no color cast (specifically, white is defined as bright and equal RGB components). The large data spike toward the right end of this histogram is a white card in the photo photographed with flash (the 5300K result). As the WB is adjusted up towards 5300, the red and green and blue components converge to equal, and white, and perfect white balance (perfect because the card is known to be right, and the equal RGB result is white). The "equal" aspect is root of the "balance" we seek. The sliders are discussed on the next page, but the Temperature slider had a large effect, the Tint slider much less here. This image is repeated here from the last example at bottom of next page.
Some definition seems required before we start. In physics, color temperature (see Wikipedia) is the color of light radiated by an ideal black body heated to high temperature. In the way that steel or lava becomes red hot at high temperature, their colors of red light are described by the temperature of the source radiator. Same idea for tungsten filaments of incandescent bulbs, and for our Sun. Other sources like fluorescent lamps are not black bodies, but their color is described as the temperature of a black body that would generate the same color of light. Black body temperature can go much higher than red, to white, and then blue. The surface of our Sun is 5800°K, which is in between the red and blue, which we call White. These temperatures are used to describe the colors of light, and color is used to describe the temperature.
The art world thinks of blue as a cool color (water and ice) and red is warm (fire or Sun). Which looks very reasonable, but numeric Color Temperature in physics is measured to be the opposite, which while the "color" blue may appear cool in art, in physics, blue is instead a higher "temperature" than red.
The camera provides the following White Balance choices to match the color of the light in our photo scene.
1D Mark IV
|Sodium vapor lamps||2700°K|
|Incandescent / Tungsten||3000°K||3200°K||2850°K|
|Warm white fluorescent||3000°K|
|Cool white fluorescent||4200°K|
|Day white fluorescent||5000°K|
|High temp mercury vapor||7200°K|
These numbers are standard ball-park guesses, a bit vague and arbitrary. But lighting varies so much, many colors exist of each, so it's doubtful we will know any precise temperature anyway. Very likely none of the choices exactly match whatever light is actually in front of us. So in the camera, we really don't have much chance of getting it closer than ballpark (other than maybe direct sunlight from our one Sun). We need to see the result first, to then see and know what it actually needs. Then it can become an easy problem.
There are two values in White Balance, Color Temperature from blue to yellow (used to correct yellow to blue errors), and Tint from green to magenta (see next Part 2 page). Adobe adds Tint to their values above (+10, except Tungsten is 0 and Fluorescent is +21, correcting green). The cameras don't mention Tint, but they do it too (note on next page).
We can set our camera to Incandescent or Fluorescent or Flash or Daylight or Shade WB (whatever, as appropriate), but the problem is, everything varies, and we don't know the actual color temperature of the specific light in front of us. Or Auto WB can take a stab at it, which is just another guess, because it also has no clue about the actual color of the light either.
Common types of photo lighting
Fluorescent lighting has some unique photography downsides.
Incandescent bulb types vary in color too, similar different types and colors (from 2600°K household to 3200°K photo, and some are towards Daylight). Color even varies with watts, and age. But at least incandescent is the definition of theoretical CRI 100. Most are Orange, but a complete spectrum, and we can easily correct for them.
Direct sunlight through the atmosphere on a clear day is about 5200°K, and pretty much the only constant color we have, always the same sun, same as expected by the camera Daylight WB. It might vary slightly with angle in the atmosphere, due to season or time of day. But of course cloudy and shade and sunsets, etc become a different story.
Flash WB is about the same color as Daylight, but flash color varies with flash power level. Also with specific flash tube, and age and use of that flash tube. Nikon recommends service for our speedlights every two years, which means flash tube replacement. Heavy professional use of course changes faster than most of us see, but the flash tube color drifts up with use, towards blue. And some flash brands start off higher. But for speedlights in general, low power flash is bluish, and high power flash is more reddish. Most studio mono-lights are the opposite, low power is more red. Sometimes portraits might not mind a bit of red, for a more warm tone, but of course, too much is a problem. Bounce from beige walls can take on sickly color, but when we get both exposure and white balance right, the good image jumps out, bright and vibrant.
Open Shade is "Open" to the sky (like at the side of the house, shaded from the sun, illuminated by the clear blue sky above) might be near 10,000°K, or maybe higher. The sky is of course very cold, and it looks blue because the red and yellow has been filtered out, but color temperature is given as the temperature of a black body that would generate the same color of light. Shade in a forest or under a cover is different, still blue, but less blue.
So light color, and proper White Balance for it, simply varies. And except for direct sunlight, we can rarely predict its color closely, we can only react to it. But we can easily react to it, to insure perfect color in our pictures.
In the old days, a good film photo lab would do much of the correction for us, both exposure and White Balance. Indoors or outdoors seemed not to matter, many labs would simply fix it for us. Reasonably well too, their goal was to sell the print, and so many of us were not even aware of a problem back then. However, digital is now our job to do this.
What about lab corrections for digital? There are many print labs and some offer options. One issue is that of course many of us do our own WB correction, and we will get angry if the lab changes what we carefully provided. I choose a print lab that offers a "no corrections" option. Other options can offer some corrections. But automatic machines do everything today, and correction simply means they let their own Auto WB have a go at it. I could have done that, but of course Auto WB failure is why many of us correct our own work. And not so many photos are printed anymore anyway.
So if good photo color is important, learning how to correct White Balance is a necessary thing to know. Fundamentally important to good picture results, it can make a really big difference. I argue that since we never know the actual color of the light, setting WB in the camera (or using Auto WB) is merely an approximate guess, If Auto WB is "close enough" is not really debatable for critical users. It seems obviously better to see and measure and correct the results precisely later. It is not difficult.
This WB fix is easiest and best done if using raw images, where we have really good tools to do it, much better than the camera has. I would argue that actually seeing what we're doing is the overwhelming advantage. It is the reason to use raw images. Frankly, if you have a WB card in a session test shot (in the same light), then you could not care less about the WB setting in the camera. You have a raw image, and you're going to set it later anyway, and WB is simply not a concern yet.
OK, there are ifs and buts... I am no fan of Auto WB, but it is halfway good, at least better. I do use Auto WB with Raw, not because it has any effect on the raw image data, but only to see the camera rear LCD preview image, and for the camera histogram, these are affected by camera WB (these features necessarily show a JPG embedded in the raw file, and JPG is affected by camera WB, and Auto WB is good enough for that). Otherwise, WB does not matter yet, there is no WB in the raw data. This camera WB setting does not affect the raw data. Instead of setting a hopeful WB in camera before the shoot, just simply set WB afterward, when you first view your shots, and can see what you're doing. There are several methods, and any try seems better than no try.
I would argue that this Custom method apples to shooting JPG, and would be good for JPG since JPG has less range to readjust it later. The method seems fiddly to do to me, and to me, Custom WB makes no sense for raw, since raw data is not affected by WB. Raw editors probably could recover WB from the Exif and copy it into the final RGB, but the point of raw is to be able correct white balance after we can see what we're doing.
White things are common in images, if you will stop and look. Not all whites are equal, an off-color wall is not a good bet. But just click some actually white object for generally good results, not fail safe, but it works pretty good very often. Far better try than nothing. The white thing must be in the same light as the main subject. If you have no other plan or opportunity, why not try it? The worst that can happen is that you might have to click Undo or Cancel, but you may like it.
Clicking the WB tool on the white thing tells the computer "This spot is neutral, make it look neutral" (meaning equal RGB components, no color cast). White works, and gray cards work too, but 18% is pretty dark to show slight color cast well (but even black can work in some cases, maybe better, if no other choice. Black and white are just extreme shades of gray, which implies no color tint). If we know the card is a neutral color, and if we click it to make it actually be neutral, then there is no color cast in the image. And it works great (unless this color was not supposed to actually look white). Exposure variations can still make it brighter or darker, and camera profiles (like Vivid) can change colored things too, but we can also see that result in raw, and easily fix that too. Just make it look like you want it to look, but precisely correct color relies on using a known neutral card. But a plain white paper or dish, or white dots on pajamas work well too, and a pleasing color result is good too. Far better than no try to fix it.
I rule out Auto WB for any serious effort that matters. It's too easy to do it better.
My own preference is clicking white things, especially a planned White Balance Card. Or even just looking by eye if necessary. But I much prefer to instead place the WB card into the scene (at the subject) for careful indoor studio work. Outdoors, I may keep an eye out for suitable white things (maybe sometimes an extra shot to include a good one if in the same light), and there is also a WB card in my camera bag for important difficult cases encountered. :)
So, while including a planned white balance card in a (test) photo is of course best for known full accuracy (more on next page), many images do already naturally contain something suitably white... maybe a T-shirt or a porcelain dish or a paper envelope. Better photo editors have a White Balance tool to click on something neutral in the image, and then the software makes it actually show as neutral (which removes the color cast from entire image). The free Irfanview or Faststone editors don't have this tool, but free Google Picasa does, and certainly the best editors do.
Raw editors, including Adobe Camera Raw, excel in this, and they can also do JPG images too. For raw images, they do have the same tools the cameras have, plus more and better, plus we can actually see what we're doing then, and can choose among different tries. Day and night better.
But another older tool first - Adobe Elements and Photoshop also have the middle eyedropper tool (marked red here) in the Levels tool (CTRL L). The same eyedroppers are in the Photoshop Curve Tool. The black and white eyedroppers set the Black Point or White Point at the tone where you click them, involves clipping and contrast (read the Help, Auto Color Correction). This center gray eyedropper is for correcting color casts, and work well for minor color shifts, not as well for major shifts.
However Raw software like Adobe Camera Raw (ACR, in Lightroom or Photoshop or Elements) has its own better White Balance tool (more modern tool, works similarly, but with much wider range - more on Part 2 page).
In the Levels tool, select the eyedropper tool (the middle gray dropper, which I marked in red here), then click a spot in image which should have been a neutral color (something neutral white or gray). Then the entire image color is adjusted so that clicked spot will become neutral (all color cast removed, based on this clicked spot actually being neutral). Preview should be on too (under that red mark), so the image immediately displays the result. The Auto button in Levels is something different (not about your neutral spot). It is another type of auto white balance (instead it aligns the right ends of the three RGB channels), assuming white there, but which does not always work very well.
Alternately, the actual method that I advocate is to use a known accurate white balance card for corrections (much of what follows here). However, sometimes we fail to include that card, but can still always expect white balance problems. Using an included KNOWN white card works best, but the point here is that utilizing whatever white things we find in the scene can be a big help then. But do realize that many things are intentionally off-white, while others may be intended to look really white.
I am NOT saying clicking everything white or gray works well, since there are many tints of white or gray which don't work. Walls are not a good bet. But many pictures include white things that are intended to look neutral white, and do work, or at least well enough to get you closer. Maybe white clouds if not overexposed from clipping. Small dots on the kids pajamas can work. Even whites of eye balls are not a bad try, so do stop and look for something white. The white thing must be in the same light as the main subject. Try a few different things, and click on a few different places on it. You will see the one you like, and will get much better at it real soon. A big improvement may not be a total fix, but it can often be much better than no improvement. Getting it close first helps when tweaking by eye too. When adjusting manually by eye, go too far one way, then too far the other way, to see the span of the middle, to be sure you see the good color pop out at you. Try both sliders. There will be a good spot.. which might be pleasant instead of accurate, but that's good too.
When serious about good color, there are inexpensive White Balance Cards which can be added to the scene in the first test shot (in same light), which provide known accurate color comparisons. Using the known source, then you know it will work, very accurately. Very convenient for fixed studio sessions with many photos under same lighting, but it works anywhere (insert card into first test shot, then remove it for final shot, and apply same correction to both). An 18% gray card works reasonably well, but is from the film era, and is designed for exposure. Its neutral color is less well controlled, and it is also pretty dark for white balance. There are "digital gray cards" of a much lighter shade of gray which are supposedly controlled to actually be neutral, and these are designed for white balance purposes. There are many such devices sold, and some seem overpriced. I like the Porta Brace White Balance Card, $5 from B&H.
Note that White Balance has two components, Temperature, yellow to blue, and Tint, green to magenta (see more on Part 2 page).
These examples are using the White Balance Tool in Adobe Camera Raw. The purpose is to compare accuracy of white things. Note that clicking on these things is very easy, not rocket science at all, anyone can do it. :) All we have to do is to choose the resulting colors that we prefer.
Examples (clicking white things in your images. A known WB white card is normally the most reliable.)
This example above was lighted with only the overhead room light, using two 13 watt compact fluorescent bulbs trying to simulate incandescent temperature. I possibly should have used Warm White Fluorescent WB for these CFL, because in this case for this bulb, Green was left too strong in Incandescent WB. That wouldn't help blue, which was weak in both Incandescent or Auto WB.
The added known White Balance card (center) was the best result of course, eliminating those concerns. But clicking on ANY of these other white areas was nearly as good - often 2600K or 2650K, and Tint often +24 or +26. Any are better than Auto or Tungsten. The chart shows their result, and the resulting White Card RGB color. White or gray is Neutral when the three RGB colors are equal (no color cast). Compared to the first two lines, the other chart results here are very nearly neutral. This of course assumes the selected white actually OUGHT to be neutral color. Clicking an off-white wall may not be a great choice.
The point is, white objects are often present, and if no white card, you really would like clicking any of these items much better than simply selecting Incandescent or Auto. You are seeking a pleasing result.
Adobe and Canon call it Tungsten WB, Nikon calls it Incandescent WB, which are the same thing (incandescent bulbs have tungsten filaments).
Perhaps you don't see much difference, and don't really care. But if you do want correct color in your images, it's not a hard effort.
|Item||White Balance||White Card RGB Color|
|Camera Incandescent WB||2850K Tint +0||(202,194,127)|
|Camera Auto WB||2850K Tint +22||(199,189,164)|
|White Balance Card||2650K Tint +24||(189,189,189)|
|Plastic cabinet||2750K Tint +22||(194,190,173)|
|Window shutter||2550K Tint +24||(184,188,197)|
|Cap of plastic bottle||2650K Tint +24||(190,189,187)|
|Painting above it||2600K Tint +25||(187,189,193)|
|White paper in basket||2600K Tint +20||(188,190,184)|
|White plate under Card||2600K Tint +26||(187,189,195)|
|Cup under Card||2650K Tint +26||(188,189,195)|
|Label on blue lid||2700K Tint +26||(192,189,185)|
|Rectangular plate, left||2650K Tint +26||(190,189,190)|
|Plastic oval lid, right||2650K Tint +28||(189,188,194)|
|Small dish above it||2650K Tint +26||(190,189,190)|
|Business card||2600K Tint +30||(187,187,202)|
|Any of these were more correct than Tungsten or Auto WB (which are just hopeful guesses)|
Daylight WB is simply wrong here (blue). Cloudy WB helped this one. Do you set that when needed?
Clicking WB on the white water splash (at red X above) is about right (make sure any clicked white is not clipping however). The big difference here is that it reduced the green cast significantly. Not bad at all, it has pluses, but I chose Cloudy WB.
The back of the white shirt (red X at left) ought to work, but it's in the shade, so it matched that shade result. So specifying Shade WB directly was about same. The clicked white needs to be in the same light as the rest of the image.
This is a RAW image, and I'm playing in ACR (Adobe Camera Raw software, Lightroom, Photoshop, Elements), but they can do same on JPG files too, maybe with less extreme range. Raw allows more range than JPG.
These are multiple choices, to look at and consider and chose after you can see the result. So when White Balance is giving trouble, look for something white in the image. However random white objects are certainly not guaranteed - if that object's true color was not actually neutral white (or gray), this can cause another color cast - maybe mild and still better, but maybe not. Even if not perfect, you can see the resulting color, and can try others around that value a bit. You would like "accurate", but you are looking for "pleasing". Often easy as pie, it just works. Sometimes not always so easy - look for other spots then. And try a few clicks around the same spot, to see if one is best. Yes, this may be hit or miss, and yes, certainly we can fault this, explaining how it does not always work, but yes, it can often be just the thing for pleasing results (consider the alternatives). If your problem picture has some color cast, and you realize there is also something white there, then why not click it? In tough situations, you can be very pleasantly surprised - but maybe not always. Then you can always Cancel out, or Undo. We don't always realize the color is bad, until we see it better, which then may be a Wow experience. There's nothing like correct white balance. You ought to go back to your old pictures, and try clicking the WB tool on some white area, just to see how this works. You definitely should try this for yourself. This is a very mainstream procedure.
It is a good plan to start with a monitor calibration system, to be sure you see things right. I still use an old DataColor Spyder 2 Express (most basic version), which seems more than enough. It is a light sensor that reads known colors and intensities on the monitor screen, and adjusts video response so that they display as expected. I bought an inexpensive Dell IPS monitor, and it's easily the best I ever had.
The Raw White Balance eyedropper tool is better than the Levels tool (wider range), but the Levels eyedropper works fairly well for minor shifts. I tend to say a white object, white seems more common, but it can also be gray, so long as it is actually neutral color - with no color cast in the original itself. Neutral colors are black, gray, and white, those pure tones with equal RGB components (no color cast by definition). Black is RGB (0,0,0) and White is RGB (255,255,255), which are just extremes of gray. Gray is any intermediate tone of equal RGB values, for example (100,100,100). But not every gray object is fully neutral color, blue and pink tints are common, which would not have equal RGB components. Even near black can possibly work here, but there is not much difference possible in channels at value zero. There is more difference detectable at higher numerical values, the same percentage of color cast is easier detected.
The card needs to be an actual neutral color. Neutral gray is a special colorless color, and white and black are special cases of gray brightness, but neutral specifically means a color with equal red, green, blue components. Certainly there are off-whites, and also blue or pink grays, but equal RGB is neutral (no color cast). We click this card in the image, and the White Balance Tool makes that spot actually be neutral (equal RGB), which removes any lighting color cast (allowed since we know that spot on card ought to be neutral). White is much brighter and shows any cast much better than black where it may be hardly detectable (near zero). An 18% gray card can work (if actually neutral), but it is a dark color, not bright.
In a fixed studio session, or even walk-about bounce in the same room (any situation with several pictures with same lighting with the subject), a truly great solution is to simply include a known true white card in the first test picture (in the same light). I normally use this Porta Brace White Balance Card. 5x7 inches, $5, plastic, durable, washable, inexpensive, accurate, quite good and it is all we need. Buy a couple and cut one to shirt pocket size. Or a WhiBal brand card is also good ($20 or $30).
Cheap cards likely have no control, and make no claims, therefore white seems easier control than colored gray ink. And alternately, even plain white printer paper (cheap copy paper) works fairly well too, perhaps better than expensive papers with added brighteners which can be blue (and the copy paper or envelope or business card are certainly far better than no try at all).
These different WB cards have different manufacturing philosophies. The Porta Brace card is just white plastic. It has no color pigments to be controlled and checked, and is very inexpensive. Probably cranked out like PVC pipe. The WhiBal card adds light gray pigments to the plastic, to be a "controlled color", which then has to be controlled, but they claim to measure and verify color of each card, to insure it has no color cast. This costs more. But this is about minor variations though, and the Porta Brace card works great too. I have a couple of each (Porta Brace and WhiBal), all are fine, they compare not precisely identical, but well enough. I normally use the Porta Brace card, it's very adequate. The 18% gray cards also do work. They use gray ink, primarily designed to reflect 18% of the light, and probably do no heroics about color control (no specs are ever mentioned). Better ones work for WB, usually well enough, but will vary a bit more, and are really too dark for WB (dark does not show color cast so well). I would suggest either the Porta Brace or WhiBal cards. I don't like the cheap three card sets (black, gray, white cards, all three with different WB).
The philosophy of use is that if we photograph the neutral card, it will take on the color of the light shining on it, and then when the WB tool clicks it, it makes makes R, G, B values be equal in the sRGB system (which is also a and b zero in the Lab system). If we click on a known neutral item, then presto, no color cast in our image. It does not get more straight forward than that. The card does have to be in the same light as your subject.
There are other fancy complicated expensive systems, but it need not be rocket science, and nothing works better, or is easier, than the white card. You click on the card in the first test image to tell the White Balance tool "This spot is neutral - Make it be neutral". And Presto! Whatever color cast there is removed, also from the entire image area (which assumes of course that the subject and background have the same lighting). The color of the clicked spot is adjusted to force equal RGB components there, which is neutral. The same color adjustment will also work when transferred to all similar pictures in the same lighting situation.
This Levels tool gray eyedropper (Part 1 page) only works on that one selected image, but at minimum, it shows you what good color looks like, a visual goal which you can then work to imitate in the other pictures. Raw editors let the one click apply to several/many selected images (selected to be in the same light). Or instead, if only the other standard photo editor color tools are available, start with the blue-yellow slider - if too blue, then try less blue, etc. Raw software is better, offering more features, specific to camera images, including actual White Balance tools, and for example, is good about copying White Balance to multiple images with one click. Also the Raw White Balance tool adjusts both color temperature (blue to yellow) and tint (green to magenta). I usually use the White Balance tool in ACR (Adobe Camera Raw, which is in Lightroom, and Elements and Photoshop). I can select all the images in the same session (using the same lighting), and click this card spot in the one test image, and the same White Balance correction is applied to all selected images at once, one click. Again, Presto! Very trivial job, but perfect results. We don't even have to think about White Balance then. It's like magic, simple and easy.
Two Rules for Using the White Balance Card:
Gray cards: 18% cards do work for white balance, but they are pretty dark because they are intended to have a reflection of 18% (for exposure purposes). They really don't make neutral gray color claims, but most are fairly close. See more about gray cards. This 18% value was believed to be what the human eye sees as middle gray, and ink press printers used the card to calibrate their ink in the old days. You should realize that other than this notion, there is nothing 50% about the 18% card, and digital sees it as 18% (when linear, before gamma). So in the 1930s, 18% was popularized as being visual middle gray by Ansel Adams in his Zone System, which was about visual results seen by eye in prints. There was no digital data back then, and 18% is NOT 50% of anything. Light meters are instead calibrated to 12.5% (Sekonic, Nikon, Canon, but Minolta was 14%). Kodak always said to open 1/2 stop if metering on their 18% gray card (which converts to 12%). Which again, is about brightness of reflection, and the ink is not specified to be neutral color. The 18% cards are pretty dark for white balance, and today the newer "digital" gray white balance cards are a much lighter shade, using color controlled inks for this White Balance purpose. White is very good for White Balance too.
Incidentally, we can still buy "Kodak" gray cards, but Kodak has not manufactured gray cards in twenty years. Kodak sold their printing business to Silver Pixel Press around 1995, who sold it to Tiffen in 2000. Both have sold "Kodak" branded cards, and Tiffen still makes Kodak Wratten filters, but apparently now they sell rights to make "Kodak" gray cards to others. Any new "Kodak" 18% gray card was not made by Kodak, and 18% cards are not necessarily exactly neutral (but are usually fairly close).
For JPG images, your camera may have the Custom White Balance feature, where you photograph something known to be neutral white or gray, a close up of a white or gray card held in the same light with subject. Or instead, you can use a strong diffusion filter on the lens, like an ExpoDisc, and aim that at the light source. The camera Custom WB procedure uses Auto WB to adjust the color of that light to be neutral (no color cast). The diffusion sort of makes a gray card from it. Pringles chips used to use a frosted can lid that was said to be really good for this. :) Either way, card or diffusion filter, this Custom plan seems good, because it does not depend on knowing the actual color of the light. It makes no actual photo.
But IMO, there seems lots to be said for a WB card simply placed in the scene for a test shot, it is an easy way. A camera-based Custom system requires you stop and do this right then, right there, every time you make a lighting change. Just sticking a white card in the scene for the first shot takes little effort, and then you can deal with it all later, leisurely. Frankly, I don't even do that in the general case outdoors. Sunlight is not that hard, and often there is already something pretty much white in many scenes. Indoor sessions however are significantly harder, many types and colors of lighting exist, even speedlight flashes vary from each other, and the white card is quite important to me.
Continued - Please visit Part 2 - More about using the White Balance card