‘Chromatics’ is the human term for the science of colour. The human eye has three types of colour sensing cells (cones) in the retina, each with a different pigment. The differing types of cone absorb differing wavelengths of light, from red (the longest) through green to blue-violet (the shortest). As each type of cone is stimulated to a different degree, different colours are perceived. As there are three types of cones, human colour vision can be referred to as trichromatic. Other animals perceive colour in a more or less complex way. Birds, for example, tend to have four types of cone cells (tetrachromatic) although pigeons are thought to have five (pentachromatic), presumably to enable them to differentiate between types of squashed pizza. Canines have dichromatic vision (allowing them to see more than black and white) as do bovines (ensuring they don’t overdose on red).
All of which made things especially confusing when America went to war with itself over the issue of skin colour.