The easiest way to think about how color vision works is to think of your television set. Your tv works by mixing three different lights (red, green, and blue) to produce all the colors that you recognize. Color vision works by using three different receptors (call them red, green, and blue) to deliver information to your brain where all the information is "mixed" and the different colors are recognized by you.
The receptors in your eye that are responsive to color are cone cells, and they are located at the back of your eye in the layer known as the retina. Rod cells are also located in this layer,
but they are bleached at very low light levels and thus do not contribute to the majority of your day to day vision. In other words, your night time vision is based on these rod cells. Cone cells are called "red, green, or blue" based on the photopigment they contain. Photopigments are sensitive to different wavelengths of light, so it actually makes more sense to call them long-wavelength sensitive ("red"), middle-wavelength sensitive ("green"), and short-wavelength sensitive ("blue"). L, M, and S are common abbreviations for "red", "green", and "blue" when refering to photopigments, so we will use these abbreviations.
This is similar to what happens when someone is called "colorblind". When someone is colorblind, there is something wrong with one of their groups of photopigments, usually the red (L) or green (M), which is why they are called "red-green" colorblind. 8-10% of men are "red-green" colorblind. This does not mean that they cannot see red or green, but that they have trouble with certain colors, usually colors that are light or desaturated. People who have normal color vision are trichromats; this means that they base their color vision on 3 different types of photopigment, one from each of the L, M, and S groups. Severely colorblind people can be dichromats;this means that they are missing one whole group of photopigment.
Most mammals (including cats and dogs) have dichromatic color vision. Monochromats are very rare and cannot distinguish colors. People who are mildly or moderately colorblind base their color vision on 3 different photopigments, but from only 2 of the photopigment groups; these people are anomalous trichromats or moderately colorblind base their color vision on 3 different photopigments, but from only 2 of the photopigment groups; these people are anomalous trichromats. from only 2 of the photopigment groups; these people are anomalous trichromats.
Colorblindness is an inherited trait. It is said to be sex-linked; this means that it is passed down on one of the chromosomes that carries sex traits. Every human has 2 sex chromosomes, women have two X's, men have an X and a Y. The genes for L and M photopigments are located next to each other (tandemly) on the X chromosome. This means that men have one set of L and M genes and that women have two sets. This is why men are more likely to be colorblind; if there is a defect in a man's genes, they are colorblind, a woman's second set of genes often keeps her from being colorblind.
This woman has a 50% chance of having a colorblind son, and if her husband has normal color vision, her daughters should have normal color vision but have a 50%
chance of being carriers for colorblindness. The woman herself is a carrier for colorblindness. If a woman's father is colorblind she is said to be an obligate carrier because she is known to have gotten the colorblind X from her father (if she had gotten the Y, she would be male!). A woman can get a "colorblind" X from her father and pass this X on to her children.