Certain types of colorblindness combined with certain colors makes text much harder to read than black-on-white. If you limit yourself to just universally readable colors, you're heavily limiting the number of different categories you can highlight. There's also the matter of screen real estate - duplicating info isn't helpful for people who can deal with colors just fine. I'm not saying it's a srong thing to do, just pointing out accessibility and usability are different things.

Couldn't you distinguish things by their grayscale value? Then you change the hue on top of that for people who can see color.