It all starts with the eye, or more specifically with the "cones" in the eye. In school, we all learned that the eye contains rods and cones, which are sensitive to light. The rods and cones communicate this sensitivity to the brain, where an image is built up that allows us to see our surroundings.
Problems with color vision occur when all the cones of one or more types are functioning below normal levels or not at all. Because the genes that affect our color vision are part of the X chromosome, color-deficient vision affects men far more than women. About 8 percent of all men have some form of color deficiency, while only about 0.5 percent of all women have color-deficient vision.