From left to right: normal color vision, mild deuteranomaly, Deuteranopia, Monochromacy (Achromatopsia)
Colour blindness is a general term that encompasses a number of deficiencies in a person’s ability to perceive colour. There are three different types of colour blindness, and varying degrees and types of each. The Greek prefixes prot-, deut-, and trit- (first, second, third) indicate red, green, or blue cones, respectively, based on their frequency (red is the most common, then green, then blue). The suffixes -anomaly (abnormality) and -anopia (absence of sight) represent either a cone that is not aligned correctly or one that does not function at all. Thus, someone with deuteranomaly would have an abnormality in the function of their green cones.
Trichromacy is when all three of the colour cones in the eye are working correctly. When one type of cone is slightly out of alignment, this is called anomalous trichromacy, and is the most common form of colour blindness with approximately %75 of colour blind people, including myself, falling into this category. Most
anomalous trichromats have what is called deuteranomaly, which is a reduced sensitivity to green light. Protanomaly is the next most common, and is a reduced sensitivity to red light. The most rare form of anomalous trichromacy is called tritanomaly, the reduced sensitivity to blue light.
When a person has one type of cone that is completely non-functional, they’re considered dichromatic. Dichromats are categorized as protanopes, deuteranopes, or tritanopes, depending on which cone is not functioning. Like with tritanomaly in trichromats, tritanopia is the most rare form of dichromacy. In fact, tritanopia and tritanomaly are about as rare as monochromacy, or true colour-blindness.
Monochromacy is the inability to see any colour at all, and is extremely rare. Only 1 in every 30,000 or so people suffers from achromatopsia.