The primary colors that the brain (not the retina) perceives are
combining these gives the secondary colors
One can see that these secondary colors are intermediate between the primary colors
One would assume then that combining a primary color and a secondary color would result in a color intermediate between the primary and the secondary. And indeed most do. Except for cyan.
Cyan is not intermediate between blue and green. It is clearly a combination of blue and something. But what that something is is not clear.
orange red-orange red magenta purple violet blue cyan 0F0 8F0 FF0 FC0 F80 F40 F00 F08 F0F 80F 00F 0FF 0F00 000F F000 00F0 0FF0 0F0 FF0 F00 00F
I suggest that there were originally four primary colors and that cyan was, and still is, between blue and the fourth primary color which no longer exists (except as a component of cyan).
The original four color receptors were
We didnt perceive the output of the green receptor as the color green. We perceived it as a fourth primary color that no longer exists (except as a component of cyan).
For some unknown reason we eventually started perceiving the output of the green receptor as between yellow and blue (as we do today). Therefore we no longer needed all four light receptors in the retina. So we dropped one. But instead of dropping the green receptor we dropped the yellow receptor.
Origin of the colors
Yellow, red, and blue are colorized, beautified versions of white, grey, and black. It isnt clear what the fourth primary color corresponded to.
white yellow grey red black blue