People perceive color by means of certain photopigments in the retina that are sensitive to certain wavelengths of light  People who are colorblind are unable to distinguish between red and green, for example, due to an absence of certain photopigments
What is difficult to explain, however, is that in a study of people who easily distinguish red from green, 10 to 20 percent failed to report distinctions between many shades of red that the majority of the subjects were able to distinguish
Some people are uninterested in fine gradations of color and fail to notice or report differences they do not care about.