What Happens When A Person Is Color-blind?

Certain colors strike the eye and fail to create the sensations which they reproduce in normal eyes. As example, rays of red light strike the eye and not only stimulate the parts of the retina that are sensitive to red, but also stimulate those parts that are sensitive to green. The green light does something in the same way. It also stimulates parts of the retina that are sensitive to red. When this happened, the watcher does not see either red or green, but instead a sort of yellowish-grey. When the colors, red and green are combined it gets a white color as result.

Such a person is color-blind only for red and green. He can still see blue, yellow and violet. But the colors, red and green always appear as various shades of yellow-grey. In fact, a color-blind person can use these shadings to tell red and green apart.