Check out this neat quiz that tests your color acuity.
It scores on a scale from 0-99, with 0 being perfect. I scored an 11, and I'm not sure which percentile that puts me in, but I feel like it's probably not bad. I have had a lot of practice with this kind of quiz, because I obsessively play Blendoku on my iPhone. (Which, incidentally, is a really fun game if you like that sort of thing.)