This is a very important topic to me. I feel that as MMO's target all sorts of gamers a color blind mode is a must as many gamers are in fact color blind in some way or another. My wife suffers from one of the forms of color blindness and it makes it hard for her to see anything that is in a shade of red. Many people think that color blindness just means they see the colors as another color or as gray/black/white but in reality it can be a bit more severe. My wife for example can't see many shades of red at all. So when enemies names are in red, quest circles on the map are redish, and enemy area of effect attack indicators on the ground are red she has a hard time seeing them.
Now as an example, World of Warcraft has two ways to deal with color blind players. One is a color blind mode which changes some icons to letters/words. The other way it does is with shaders that add a color overlay over the screen based on the different forms of color blindness. (Continued)