I think this tip is probably the most important: "Be sensitive to what sort of information the individual needs. Always ask how they think of colors, as they will know how it's best described to them. Chances are they're used to being blind and know best how to deal with such things." See more tips to help describe colors to a blind person here: http://www.wikihow.com/Describe-a-Color-to-a-Blind-Person
Its important to note there are various kinds of blindness with the most extreme being "complete lack of form and visual light perception." http://en.wikipedia.org/wiki/Blindness So, some people can sense light, which gives them a starting point... They may recognize that red is darker than say blue or yellow.
To describe colors to a blind person, you need to put it into a perspective that he can feel. The color red might be something hot, blue might be water, white might be ice, and so forth. It doesn't really help them to know what it looks like, but it gives them a sense of what the color feels like to use when we see it.