If you're wondering what color-blindness is, you can start reading this Wikipedia article. I seldom notice the effect of such an impairment and when I do, I'm usually interacting with a computer.
There exist many forms of color blindness and mine is called protanopia. Protanopia differs from other forms, such as deuteranopia, because we experience an abnormal dimming of light at some wavelength with the result that, for example, red is easily confused with black.
In the case of the iPhone, I must admit that I'm experiencing just one problem: missed calls. There's a fancy icon down there indicating the number of unchecked missed call. When you open the corresponding window, guess what? Missed calls are "highlighted" using the good ol' red fonts while received or dialed calls are black. The background? White, just to make it worse. I tried looking for some other color scheme but there's none. Just the good ol' "iPhone experience" that Apple is providing us.
This rare impairment affects almost 1% of the male population. Not much, indeed. But for us, there are no red characters to catch our attention. They just fade to black, hiding between the others. If you're working with a color-blind person, please follow good sense: don't rely on colors and please, do not use red to catch their eyes. Just rely on the tools your word processor is offering you, such as that yellow marker whose existence you probably wondered about sometimes.