Color blindness is most often the inability to distinguish between red and green. Less often a person will have problems with blue. A common misconception is that color blindness is the complete inability to see any color and that the person in question can only see white, black and shades of grey. This is simply untrue. Many color blind people can see the specific color, they just have a hard time distinguishing it from other colors. Only one in one thousand cases of color blindness are people who truly see in monochrome.
The cause for color blindness is genetic, and it has to do with the rod and cone light receptors in your eye, most people have 74% red cones, 16% green, and 10% blue. People with color blindness may have a reduced amount of receptors or in come rare cases no receptors in the specific color at all.
Basically to learn more (http://www.toledo-bend.com/colorblind/index.html), and to take a color blind test (http://www.toledo-bend.com/colorblind/Ishihara.html). They also have color swatches of commonly accepted colors and their names. Enjoy.
Info from (http://channels.netscape.com/ns/atplay/colorblind.jsp)