Trichromacy is a term that refers to normal human color vision. Human vision partly depends on the retina, a thin sheet of tissue on the rear inner surface of the eyeball. The retina contains two basic types of sensor: rods, which sense brightness, and cones, which sense color. The average human retina conains about ten million cones. The cones are divided in function according to the color of light they sense, being the three primary colors, red, green and blue. Normal trichromacy occurs when the three types of cones are equally able to sense their color of light at the same intensity. Anomalous trichromacy is a type of color vision defect in which one or more of the types of cones are less sensitive, leading to imbalances in the ability to see color.
Anomalous trichromacy is distinct from dichromacy, in which one type of cone is absent, and monochromacy, in which two types of cones are absent. In anomalous trichromacy, all three types of cone are present, but they do not function equally well. It is estimated that five percent of the male population suffers from anomalous trichromacy, with the disorder being somewhat less common in women. The most common deficiency is in the green-sensing cones, leading to difficulty in distinguishing green from red or, less commonly, green from blue. The second most common is red-sensing cone deficiency, usually resulting in difficulty distinguishing red from green. The least common type of anomalous trichromacy affects the blue-sensing cones, usually creating difficulty in distinguishing blue from yellow.
According to this website, the usual ascribed cause of anomalous trichromacy is low levels of pigment in one type of cone. The website proposes that the more likely cause is an abnormality in the pigment itself, which is present in sufficient quantities but not properly balanced in terms of the wavelength of light it is most sensitive to.
The theory of color vision used today was first proposed in the 19th century and updated in the 1960s. Under this theory, white light is a combination of lights at various wavelengths that would normally be seen as red, blue and green, being combined at the same intensity. Colors such as yellow as the result of two but not all three of the wavelengths of light being dominant in intensity. This theory is different from the theory of light used to discuss reflective rather than light-emitting surfaces; in reflected light, blue, green and yellow are the primary colors.