The eye has two types of sensitive cells in the retina, the rods and the cones. The rods are sensitive to lightness and darkness and the cones to color. There are three types of cones within the retina, one sensitive to red, another to green, and the third to blue.
The cones send a signal to the brain that sets up a response in terms of opposing pairs. One pair is red-green and the other is blue-yellow, and this is why we have individuals who are red-green or blue-yellow color blind. There are no individuals who are blue-green or red-yellow color blind.