Retinas have things called cones. These are what you use to see colors. There are three varities of cones red, blue, and green. (or at least are commenly referred to as such, see retina for more information.) The way your retinas are desensitized, or more specifically the way your cones are desensitized, is by abstaining from a certain color for some duration.

For example: I was down in my room playing Civilization II. The computer screen on my computer is messed up, and the red doesn't not come in at all. Thus when I see a barbarian in the game, that is usually noted by a red color, it is actually black. After about an hour of not seeing red, I came upstairs and logged on to Everything and all that had previously looked white now looks pink.

This phenomena happens because I abstained from the red cones in the retina for a long time. In theory, if I would have seen mass amounts of red for a long duration of time then things slighty pink would appear white.