If shooting flash indoors with mixed ambient lighting, you have to pick one of the two choices about white balance... We cannot mix two colors of lights (flash with incandescent or fluorescent), so do you want the room lighting to contribute into your picture, or do you want to minimize the room lighting white balance problem?
If you don't want the continuous room light messing with the white balance in your flash picture (easy way - subject of a previous page), then simply use low ISO and the maximum shutter sync speed to keep it out, so Flash White Balance can be used (high ISO indoors boosts the orange incandescent ambient, but reduces the necessary flash power, emphasizing the orange). Faster shutter speed does not affect or reduce the flash, but it does affect and reduce the continuous ambient. 1/200 second or so will really knock down the ambient in most cases indoors. A lower ISO, or a smaller diameter aperture (f/5.6 or f/8 instead of f/2.8) will increase this effect (this ambient reduction at 1/200 second), but that will require more flash power, which must be available. Studio lighting will always use low ISO AND maximum shutter sync speed to keep the continuous ambient out of the controlled environment. Even the 150 watt modeling lights in the close flash units cannot survive f/8 at 1/200 second at ISO 100 (we should verify that the same picture exposure without the flashes does come out totally black).
If you do want the room light to contribute in your picture (assuming it is bright enough, or ISO is high enough), then incandescent lights are orange, and fluorescent lights may be green, while the flash is daylight (white). Sometimes we like a mild orange warming, but more can be ghastly too. You cannot process with two White Balance values for mixed lighting, you have to choose one. Meaning, we have to make the light colors match. We can use colored filters on the flash to convert the flash to be orange or green too, and then set camera white balance for incandescent or fluorescent, to match the ambient lighting. If you are using high ISO (or Auto ISO) to balance indoor lighting with flash, then keep reading below, you definitely need a filter on your flash, to be able to balance color too.
Said again - to combine orange incandescent room light with daylight flash: We can put a CTO filter (Color Temperature Orange) on the flash head, to make its light orange too, same as incandescent. Then we can properly set Incandescent White Balance in the camera or Raw, and be good to go.
Below are a few Rosco color filters from the (near free) Roscolux Swatch Book - and there are other choices too. The significance is 1) these are sized to fit a speedlight as is (attach with masking tape), 2) they are near free, and 3) a vast variety of choices are provided. Descriptions and spectrums of these swatch books are Roscolux and Cinegel. These Rosco Swatch books may be fully free from your local camera store if you ask about them... they are samples. These swatch book samples include almost every filter (the stack is about two inches thick), and are suitable for taping on a speedlight - just not directly against the flash lens (see below). The Rosco Cinegel Swatch book is "only" 1 3/8 inch thick, but it contains these same standard filters too. B&H says these are 2.75x1.75 inches, but mine are 3.25x1.5 inches. The color filters are polycarbonate or polyester plastic today, but they are still commonly called gel filters, or gels, from when gelatin was used.
Note: the printing below is on a small white paper separator that comes between each clear filter.
Incandescent: Any one specific room lamp can be one of several shades of these colors, so these conversion filters come in several shades too. Selection may require some testing to match your lights (but close is better than none). These orange filters are named CTO (Color Temperature Orange) to match flash to various incandescent lamps. Incandescent lamp types vary, but start with 3/4 CTO to convert flash or daylight to match the lamps (and then use Incandesent White Balance). This CTO filter on the flash makes it be orange to match the incandescent lights, and then Incandescent White Balance in the camera works (any daylight coming in from windows will then appear blue). The CTS set (Color Temperature Straw) are popularly used instead of CTO, being slightly less red, sometimes better for human skin. Or oppositely, CTB (Color Temperature Blue) filters can go on the lights themselves to convert tungsten to match daylight white balance (similar to the blue flash bulbs of yesterday). The terms tungsten and incandescent mean the same thing regarding white balance, but there are differing colors of these lamps.
These filter densities can be stacked, for example, stacking 1/2 CTO + 1/4 CTO makes 3/4 CTO, which is why they are named this way. These filters reduce the effective flash power somewhat, perhaps 0.3 EV to 1.0 EV (Transmission 50% is a one stop loss).
Fluorescent lamps have a non-continuous spectrum, hard to match completely. Fluorescent has a green component, so PlusGreen filters (above) are used on the flash to match flash to various common fluorescent lamps (so fluorescent white balance can be used). Colors of different types are:
Shutter speed issues with flicker: Fluorescent lights are continuously on, except they are powered with AC. Using the old magnetic ballast, they flicker with the AC line frequency - twice per cycle of 60 Hz in North America (120 times a second), 50 Hz in many other places. The 60 Hz AC voltage swings plus and minus, passing through zero 120 times a second. This is a big problem for shutter speed duration, which can randomly sample partial cycles and see varying color shot to shot (often this problem randomly sees a strong brown cast). I am speaking 60 Hz, but the solution is in setting the shutter speed to match the AC line frequency, either to 1/60 second, or double at 1/120 second (in North America, or 1/50 or 1/100 second in 50 Hz locations) helps to see only complete cycles for more stable color. Slower even fractions of 1/60 second work OK (like 1/30, 1/20, 1/15, 1/10 second), but faster shutters, like 1/80 or 1/100 or 1/400 second, would be a bad match, capturing incomplete cycles of flicker.
However, modern electronic ballast (since 1990's) has changed this situation greatly today, which converts fluorescent operation to about 20,000 Hz - for no flicker (in newer light systems). CFL bulbs, and desk lamps too, use the electronic ballast. And many new ceiling light installations too, but the old magnetic ballast is still available. I recently replaced an ordinary two 40-watt tube fixture, and the 25 year old ballast was magnetic, and the new ballast was electronic (and much smaller).
You can determine if your florescents flicker, this way: For this, don't use Auto WB. Shutter speeds of 1/60 or 1/120 second should match the cycles, but you are seeking flicker, so any intentional faster shutter would be good, or rather bad - it captures incomplete cycles of the flicker. 1/250 second at 60 Hz captures 120/200 = 48% of a cycle, incomplete cycle for old magnetic ballast, incomplete color, which varies randomly from shot to shot. Or maybe 1/200 second for 50 Hz lights captures 50% of a cycle. But 1/200 second captures many complete cycles for newer electronic ballast, for complete color. You can aim the camera directly at the lamp for this test, for brighter light to achieve faster shutter. Take several, 5 or 6 all the same, without Auto WB to mess it up. If results vary dramatically (a few are randomly brown), that's flicker. If all of several results are the same color, no flicker. If all the same, but color is off, that's just White Balance, not flicker.
But regarding White Balance... Even if the lamp is called "Daylight", no fluorescent has a continuous color spectrum, which makes matching White Balance be a special problem, an exact match is never possible. Fluorescent lamps have a CRI rating (Color Rendering Index). See the spectrum test on this link, shown by single slit diffraction - discrete lines vs. continuous spectrum. When choosing new bulbs, a high rating (CRI 80+) has better color - not perfect, but better, often nearly acceptable, but a lower CRI is poor for color photography (and poor in the closet where your wife selects her clothing - she wants high CRI too, CRI 80+). Perhaps most colors look OK, but the colors not in its spectrum won't look right. In contrast, incandescent lamps do have a continuous spectrum and they are the definition of theoretical maximum CRI 100. Incandescent may be orange, but it can easily be matched, all of the spectrum is present (tungsten filaments). Direct sunshine is continuous too, but fluorescent and LED lights simply are not continuous spectrum.
Sodium vapor and Mercury vapor lamps have similar flicker problems, and much lower CRI ratings than fluorescent bulbs.
Nikon furnishes a few color filter sets, see Nikon SJ Speedlight Flash Color Filter Sets. The red filter on bottom flash is from SJ-1. My best guess is that the Nikon filter numbers approximately match Rosco this way:
This match is close, but we don't know the color of our lamps anyway, lights vary too.
Rosco also packages 55 of these filters into a Strobist Kit. It includes multiple copies of a few of the important colors. Same 3.25 x 1.5 inch size, but without the punched hole, which was for the swatch book.
These filters come in larger sheet sizes, like 20x24 inches for around $6, but the Rosco Swatch Book offers many choices to see and try, suitable for speedlights. The CTO filter from the Rosco Swatch book is shown here at top on a SB-800 speedlight, simply using masking tape to attach the filter to the flash. But with a slight bow, so it is not touching the lens, so it does not melt at full power. Shown at bottom is a Nikon Red filter, which has been a little too close to a full power flash at time or two. Doesn't affect its light, but just a slight air spacing is surely better (bow it slightly, as shown on CTO Rosco at top). After you figure out what filters you may need, then you can do a more elegant mounting if desired - or not, masking tape works fine too.
You can use the filters to change your flash to match the ambient light, to use its white balance. You can use the filter to change the color of your portrait background (medium gray backgrounds work best for this). I have used the filters to add color to water drop splashes..
Changing subject now, no filter: Flash tubes are generally the same as daylight color, but speedlight color varies significantly with the power level used, more red at high power (far or bounce), and more blue at low power (close or direct). In the Nikon and Canon systems, the speedlight knows the power it actually used, and if on the hot shoe, and if Auto White Balance is selected (allowing WB value to be changed), it can report an actual color temperature to the camera (called "Flash Color Temperature Information", see sample). This is a very special deal (hot shoe flash totally changes meaning of Auto White Balance - but only works in TTL BL mode), and this information can be used for a more accurate White Balance temperature. Whereas, if you instead set Flash White Balance, that is what you get, but it is a constant correction for any picture. Still close maybe, but the speedlight flash color varies with power level.
However (just my opinion), otherwise, other than with the system's hot shoe speedlight flash, Auto White Balance is a mixed bag... it tries to analyze the picture results, and shifts the three RGB channels to have equal brightness levels. That may or may not be correct, it depends on chance (scene content, light color, etc). So then (without any Flash Color Temperature Information reported), Flash White Balance seems better, at least it knows the light should be generic flash color (which is about the same as daylight).
Colored backgrounds can be created with these colored filters too. We can create red, green, blue, yellow or fuchsia backgrounds, with a filter on the flash that is illuminating your studio background. The filtered light will change the color of the background, perhaps to better compliment your subjects clothing color. A medium gray background is great for this. If the background is white, you can only use minimum power level (more power just makes the background more white), and your main light may wash it out unless it is way back, and you probably will get only pastel colors. If your background is black, you can get intense shades, but it requires very high power level to turn black bright. Keep its distance back far enough so there is no risk of colored light reflection onto your subject.
A middle gray background is a very good compromise, very practical. However, on gray, the filter colors will come out a strong pure red or green or blue, arguably a bit much color for portraits. White background paper gives more subdued pastel colors with filters.