Four primary colors

The primary colors that the brain (not the retina) perceives are


 * yellow
 * red
 * blue

combining these gives the secondary colors


 * orange
 * purple
 * green

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 produced by combining blue light and green light but the color we perceive is not intermediate between blue and green.

The color we perceive as cyan is clearly a combination of blue and something. But what that something is is just not clear.


 * {| border=2 style="border:1px solid black; border-collapse: collapse; "


 * -style="text-align:center;"
 * green
 * yellow- green
 * yellow
 * orange- yellow
 * orange
 * red-orange
 * red
 * magenta
 * purple
 * violet
 * blue
 * cyan
 * style="width:50px;" bgcolor=00FF00 |
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 * style="width:50px;" bgcolor=FFC000 |
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 * style="width:50px;" bgcolor=FF4000 |
 * style="width:50px;" bgcolor=FF0000 |
 * style="width:50px;" bgcolor=FF0080 |
 * style="width:50px;" bgcolor=FF00FF |
 * style="width:50px;" bgcolor=8000FF |
 * style="width:50px;" bgcolor=0000FF |
 * style="width:50px;" bgcolor=00FFFF |
 * 0 F 0
 * 8 F 0
 * F F 0
 * F C 0
 * F 8 0
 * F 4 0
 * F 0 0
 * F 0 8
 * F 0 F
 * 8 0 F
 * 0 0 F
 * 0 F F
 * colspan=12 bgcolor=yellow |
 * 0 F 0 0
 * 0 0 0 F
 * F 0 0 0
 * 0 0 F 0
 * 0 F F 0
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 * 0 0 0 F
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 * 0 0 F 0
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Retina


I suggest that there were originally four primary colors and that cyan was, and still is, intermediate between blue and the fourth primary color which no longer exists (except as a component of cyan).

The original four color receptors in the retina were


 * green
 * yellow
 * red
 * blue

Originally 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 we do today as green (between yellow and blue). 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. Today we only perceive yellow when both the green and the red receptors are activated.

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.


 * {| class=wikitable


 * white
 * yellow
 * grey
 * red
 * black
 * blue
 * }
 * blue
 * }