This site is pretty interesting. You can upload an image and find out what it looks like to a color-blind person. It's worth keeping in mind when designing web pages and other imagery. I read that over 5% of males are red-green color-blind (and 1% or less of women). So that's not an insignificant number of people.
A little further reading online and I found that there is complete color blindness which is called achromatopsia. Fortunately the incidence of this problem is far less (1 in 33,000 in the U.S.). Although the incidence is over 5% among the natives of Pingelap Atoll in Micronesia due to a small genetic pool centuries ago. There is a documentary about it called The Island of the Colorblind which sounds interesting. I'll have to watch for it.