Color blindness is an inaccurate term for a lack of perceptual sensitivity to certain colors. Absolute color blindness is almost unknow. There are three types of color receptors in our eyes, red, green and blue. We also have black and white receptors. They are more sensitive than the color receptors, that is why we have poor color perception in the dark.
Color deficiency comes as a result of a lack of one or more of the types of color receptors.
Most color perception defects are for red or green or both. Red-green color deficiency is a result of a lack of red receptors. It is also possible to have the color receptors missing entirely, which would result in black and white vision. The earliest years of education use color as a teaching method and colors such as red, yellow and green are heavily employed. Many who have color vision defects are unaware of their problem especially grade school children.