People with normal color vision have three types of cones--one to receive long waves, one to receive medium-length waves, and one to receive short-length waves. These cones perceive reds, greens and blues respectively and are often labeled "L," "M," and "S" cones. Once the eye structures receive these wavelengths, the brain determines what color to see by evaluating the ratio of signals from each cone. In colorblind people, a cone is either missing or defective. A missing cone means that particular type of wavelength cannot be perceived. A defect in the cone can throw off wavelength perception, skewing what color is seen.
Anamalous trichromacy is caused by a shift in sensitivity of one or more of the cone pigments and is the most common type of colorblindness overall. That means those with anamalous trichromacy perceive colors, but they do not see them as normal color vision people would. Red weak colorblindness, or protanomaly, and green weak colorblindness, or duetanomaly (specific types of anamalous trichromacy) make up 99 percent of all colorblind cases. They are usually caused by a genetic defect on one of the sex chromosomes. Someone with anamalous trichromacy that is "red weak" may see a red apple as an orange one.
The vast majority of colorblind cases are due to genetics. Most of these genetic defects affect the "L" (or red) cones and the "M" (or green) cones. Sometimes, the "S" (or blue) cones are affected, resulting in yellow/blue colorblindness called tritanomaly. While the first two are sex-linked, the last is not.
In some instances, an accident or disease causes colorblindness. Cones can suffer loss of function due to a disruption in neural pathways between the eye and vision centers of the brain. Brain damage and diseases like Parkinson's can also produce symptoms of colorblindness.
Men are far more likely to be colorblind than women from birth. Between 5 to 8 percent of men are colorblind, while only 0.5 percent of women have this abnormality. Women are far less likely to suffer colorblindness because the most common types of colorblindness are caused by defects on the "X" chromosome. Males have an "X" and a "Y" chromosome, while females have two "X"s. In order for colorblindness to present in women, both "X" chromosomes must have the same colorblind defect. Men must only have the defect on their one "X" chromosome. This also means women are much more likely to be carriers of the anomaly than someone with symptoms of it. Tritanomaly (yellow/blue colorblindness) is the exception because the cause of this colorblindness is a mutation on a chromosome present in men and women in equal numbers.
Very rarely, someone will either be born with, or acquire through accident or disease, a form of colorblindness called monochromasy. Monochromasy is the lack of perception of any color. Those with this condition see black, white and shades of grey.