Purple was a colour.
The Thin Purple Arc was one of two epochs in the lives of the residents of the planet Fragrance. It was the time from a person's birth to when they fell in love. (AUDIO: The Fragile Yellow Arc of Fragrance)
Celery turned purple in the presence of gases in the Praxis range of the spectrum. One of the reasons the Fifth Doctor wore a stalk of celery in his lapel was to detect such gases. (TV: The Caves of Androzani)
In comparison to other coloirs like red or black, purple and yellow were inferior psychological triggers in humans. As such, a Purple Eye of Eternity never appeared in Through the Eye of Eternity. (PROSE: The Book of the War)
Omphalos Prime was described by Leabie Forrester as a cold, purple world. (PROSE: Dependence Day) The planet Quinnis had a purple sea. (COMIC: A Religious Experience) The sky of an Earth-like planet was purple. (PROSE: Follow the Phantoms)
Ukkans were light pink-skinned humanoids with enlarged purple craniums, around a dozen purple tendrils on the head, and purple lips. (TV: Lost Library of Ukko) Equinoids resembled purple horses with yellow spots. (COMIC: Perceptions) The Xyz were various shades of purple. (PROSE: Not So Much a Programme, More a Way of Life) The Huducts had purple skin. (TV: Curse of Anubis)