About 7% of men suffer colorblindness of one sort or another; very few women do. (It's higher among Caucasians than among Asians or Africans.)
Of those who have some color blindness, the overwhelming majority are on the red-green scale (they're either red deficient, or green deficient, but either way, then end up unable to distinguish red from green.) A very, very small fraction fall on the blue-yellow scale, or see no color at all - 90% of the people with a color perception issue are on the red-green scale.
The following is a generally safe set of four colors across all types of color blindness:
(White or Yellow), (Black), (Blue), (Red or Green.)
You can get a way with using red and green as long as the red is a deep red, not an orange-ish red. Given a red and a green, people with red-green problems can often tell them apart, but won't be able to tell you which is red and which is green (they'll often report them both as different versions brown and tan)... but they'll likely be able to distinguish the two as different, as long as they aren't close to yellow. (The orange zone is trouble.)
This kind of chart gives you an idea of the trouble spots. It's not really how a color-blind person sees things, but it does tell you what things aren't differentiable by a particular kind of colorblindness:
Choose any two colors in the Normal spectrum, and draw straight lines down through the other spectra; if the the two colors on the other chart appear similar, those kinds of color-blind people will have trouble. For example, 600 and 525 are obviously orange and green if you have normal vision, but those same frequencies, if you have protanopia, look similar.
