Red–green color blindness affects up to 1 in 12 males (8%) and 1 in 200 females (0.5%). The ability to see color also decreases in old age. In certain countries, color blindness may make people ineligible for certain jobs, such as those of aircraft pilots, train drivers, crane operators, and people in the armed forces. The effect of color blindness on artistic ability is controversial, but a number of famous artists are believed to have been color blind. Signs and symptoms
Color blindness describes both a symptom of reduced color perception, as well as several conditions where colorblindness is the primary – or only – symptom. This section will focus only on color blindness as a symptom. A colorblind subject will have decreased (or no) color discrimination along the red-green axis, blue-yellow axis, or both, though the vast majority of the colorblind are only affected on their red-green axis. The first indication of colorblindness generally consists of a person using the wrong color for an object, such as when painting, or calling a color by the wrong name. The colors that are confused are very consistent among people with the same type of color blindness. Confusion colors
Confusion colors are pairs or groups of colors that will often be mistaken by the colorblind.
John Daltons original hypothesis for his deuteranopia was actually that the vitreous humor of his eye was discolored:

I was led to conjecture that one of the humours of my eye must be a transparent, but coloured, medium, so constituted as to absorb red and green rays principally... I suppose it must be the vitreous humor. An autopsy of his eye after his death in 1844 showed this to be definitively untrue, though other filters are possible. Actual physiological examples usually affect the blue-yellow opponent channel and are named Cyanopsia and Xanthopsia, and are most typically an effect of yellowing or removal of the lens. Tetrachromacy in carriers of CVD
Females that are heterozygous for anomalous trichromacy (i.e. carriers) may be tetrachromats. These females have two alleles for either the OPN1MW or OPN1LW gene, and therefore express both the normal and anomalous opsins. Because one X chromosome is inactivated at random in each photoreceptor cell during a females development, those normal and anomalous opsins will be segregated into their own cone cells, and because these cells have different spectral sensitivity, they can functionally operate as different opsins. This theoretical female would therefore have cones with peak sensitivities at 420nm (S cone), 530nm (M cone), 560nm (L cone) and the fourth (anomalous) cone between 530nm and 560nm (either M* or L* cone).If a female is heterozygous for both protanomaly and deuteranomaly, she could be pentachromatic.