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Unique Colours
Unique colours or hues were first described by Ewald Hering in his theory of Opponent Processes.
He defined the unique hues as red, green, yellow and blue, and based them on the concept that these colours could not be simultaneously percieved.
For example, a colour cannot be both red and green, because they would cancel out to yellow.
There is a great deal of variability when defining unique hues experimentally, but a single observer can set their experience of a unique hue extremely consistently, and to within a few nanometers.