Why 3 colors?

Light has 2 degrees of freedom;

(frequency,amplitude) ---> (color,intensity)

All colors saturate to white in high intensity and to black in low, thus defining the evolutionary boundary. Color helps navigation in demanding environments like tree tops and expands in the gray middle range as it evolves. It works as an indexing agent for space, therefore there are 3 colors.

If fast navigation in a 3D environment is an evolutionary selection factor for the species, then they would die off if there were no possibility of extending the gray vision. But this possibility exists since there are 2 degrees of freedom of light.

The vision of 3 primary colors creates a 3D color matrix of the observed scene and maps it to the gray vision of intensity. This creates a 3D addressing mechanism and defines an improved indexing and addressing of the field. Senses are defined by their dedicated organs, like hardware peripherals of computers. Dedicated organs are needed for faster detection.

Gray on/near gray is still gray, but two different colors with the same grayness (intensity) stand out when near each other, and produce a different color when overlap due to two eyes. This improves the stereographic vision. Otherwise you need to swing your head side to side to improve the depth of the field. This is well observed in many animals, and it is a security risk and a time waster.

"Why do dogs have none?"

Assuming that the dogs have no color vision, it would be because they do not need to navigate in a 3D environment.

"Why do some humans with eyes just like yours, still not see any color
or only 2."

All genetic traits have a statistical distribution. Most humans have good color vision.

...

PS; The above is an extract from an exchange at sci.philosophy;

http://groups.google.com.au/group/alt.philosophy/browse_frm/thread/ccddf056eaf7e793?hl=en#