I thought this could be referring to :
… where purple is considered to not be included within the spectral bounds of infra-red ↔ ultra-violet, but is concocted by the brain to reconcile the circular space that having three color receptors implies. (i.e. it has properties of single frequency light (at least perceptually), but no one frequency produces it - as though there was a pure tone that wraps around from the lowest note to the highest in sound)
First recall encountering the idea in the book Elemental Mind by Nick Herbert.
