One of the common questions people have is what it means to be colorblind since that is one of the common visual deficit manifestations that they can relate to. Color blindness (pretty good article on Wikipedia there) is very common and just about everyone knows someone who does not see the world quite like they do because of a form of color blindness.
I have a post over on Jonesblog that renders some natural scenes as protanopes, deuteranopes and tritanopes would see them and talks about how to simulate your own images for various forms of color blindness.