How does color blindness work? (Question by brokenglass)
The type of neuron that processes light in the eye is the photoreceptor, and they come in two types: rods and cones. In the top picture above, you can see these two types. Cones are the ones that process colors (rods are more light/dark, non-specific, movement detectors, etc.). In these photoreceptors, there are opsins that detect some wavelengths of light better than others. In the lower image, you can see the wavelengths that the three cone opsins can detect and the much broader range that the rod opsin (rhodopsin) detects.
These opsins absorb the wavelengths of light and essentially excite the photoreceptor. By having all three, you are able to detect a whole range of colors. Some will excite certain opsins more than others, etc., and they work together to provide an accurate shade of color for whatever you are looking at.
In color blindness, the person is lacking one or more of the cone opsins. They can still see, but they are unable to detect that range of colors and discriminate those shades as well. Since the gene that makes these proteins is on the X-chromosome (women have two X’s and men have one X- paired with a Y), men are much more likely to be colorblind. If a woman does not have all the genes for all three color opsins on one X-chromosome, it is likely that she has the missing one on her other X-chromosome. For men, if they are missing those genes, they cannot get them from another chromosome, since they only have one copy of the X-chromosome. That’s why you are more likely to come across a man with color blindness than a woman.