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 blindness 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. About 10% of males have a color perception defect, but this is rare in females. Red-green color blindness is a result of a lack of red receptors.
Another form of color blindness -- yellow-blue is the second most common form, but it's extremely rare. It is also possible to have the color receptors missing entirely, which would result in black and white vision.
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