According to the opponent color theory, the perception of color is based on the output of cones, each of them an opponency between colors.

1. Three cones2. Two colors

Explanation:

The opponent process theroy of color vision recommends that our capacity to perceive color is constrained by three receptor complexes with contradicting or opposing activities. These three receptor complexes are as follows: Dark white complex. Blue-yellow complex. Red-green complex.You don't see greenish-red because according to the opponent process theory, the opponent cells can just identify each of these colors at a time.