I was surprised when I went to the eye doctor a few years ago for new glasses and they ran the test(I had no idea what it was) and then told me I'm colorblind. In retrospect it made sense, as people teased me about what I clearly though were the right colors(I remember an incident in school using green sharpie and thinking it was black). I'm red/green colorblind, but I mostly have problems with telling subtle differences, rather than outright not seeing green or red. I can see red, my cup is red. But if I were to put two colors that aren't too far apart in hue(a close shade of yellow and green) I'd start having a problem.
For example, when I first submitted this piece:
http://www.pixeljoint.com/pixelart/68501.htmI just straight ramped the shades rather than adjusting the hue:
I got a lot of complaints that it looked green, it completely caught me off guard. I often have friends double check my colors now to make sure my yellows aren't green, and my orange picks are actually orange.