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Hue opponency: A constraint on colour categorization known from experience and experiment

The terms red, green, yellow, and blue are both necessary and sufficient to describe our chromatic experience. Their uniqueness and opponent nature is supported by evidence obtained under supra- threshold conditions, especially hue cancellation. These constraints are nontrivial. How some electrophysiologically identified mechanisms contribute to colour appearance is not known, but their complexities do not refute our experience of elemental hues.