This is my understanding of color vision in humans:
Our retinas have two general types of photosensitive cells: rods, which are sensitive to light intensity, and cones, which are sensitive to light frequency. There are three types of cones in the human eye, and each one can be characterized by the degree to which it responds to different frequencies of light. One of them is most sensitive to what we call “red”; one of them is most sensitive to what we call “green”; and the last is most sensitive to what we call “blue.” (This is why red, green, and blue are the “primary colors of light” as we perceive them.) The ‘red’ cone contains a photosensitive chemical that is most sensitive to red frequencies, somewhat sensitive to magenta and yellow frequencies, and not sensitive to other frequencies. The frequencies between green and red, which are shades of yellow, activate both the ‘red’ and ‘green’ cones to varying degrees. We also perceive as yellow that light that is comprised not of a single frequency between red and green, but of both red and green frequencies. [Similar explanations for other types of cones/colors]
There are other animals that can see in color, and I’m sure many of them (most chordates that can have color vision, I reckon) have optical physiology/anatomy very similar to ours. I have a few questions about these animals:
A
[ol]
[li]Are there any whose primary colors of light are significantly different from ours? Surely there are other good photosensitive chemicals, that respond to different frequencies of light, than just the ones in human eyes.[/li][li]Are there any that see light as having more than three primary colors? This would presumably result in a much more vivid visual field. (With 3 primary colors, we can make 3 secondary colors by combining 2 at a time; with 4 primary colors, for example, we would be able to make 6 secondary colors by combining 2 and 6 “tertiary” colors by combining 3 at a time.)[/li][/ol]
And I have a question about animalian color vision in general:
B
What animal has the “best” color vision? I think the answer should take into account the range of visible frequencies (I know there are some animals that can see what we call “ultraviolet” light, for example), and the number of components into which they can decompose a sample of light consisting of (maybe a continuum of) different frequencies (for example, we can decompose any sample of incident light into red, green, and blue components).
This is not a homework assignment; I just got curious about this subject 
I have a copy of Gray’s Anatomy in my house, but apparently even human color vision was poorly understood when it was published (1970’s, I think).