Here is another example, scanning text on a sheet printed on dark green paper, really dark green. First, here's the color scan to show what it really looked like.
Setting the histogram White Point
aggressively would not be appropriate here,
because there is no color that should be as bright as white. I set the
Black Point normally, but only went a little way with the White Point,
judging it by eye to make it be a little brighter. The paper actually
looked more like the "before" step.
In the Line art scan, it seemed like the scanner must surely be defective, because the Line art preview image simply showed an all black page.
Looking at the histogram, it appears all of our pixels are below the
threshold. That is the definition of black in Line art.
Moving the threshold to the left, to be below the bulk of the green paper background color pixels makes it be an all new ballgame.
This setting gave an excellent copy (below at 300 dpi). The threshold is set so that the pixels of the dark green background are on the right of the threshold, and so they become white. The smaller peak of the fewer black text pixels are kept on the left of the threshold, and they remain black.
If we were trying to OCR text on this dark green background, this same threshold technique would make all the difference, great success instead of dismal failure.