Scriabin assigned each key a color, C red, G orange, D yellow, onward around the circle of fifths through F sharp’s bright blue. The geometry is the tell: neighboring keys get neighboring hues, which suggests a constructed system as much as raw perception, and scholars still debate how much was genuine synesthesia versus theosophically flavored design.
His contemporary Rimsky-Korsakov kept a rival chart, and the two famously disagreed on almost every key, a clean early demonstration that key-color mappings are personal rather than universal.
The system matters less as psychology than as practice: Scriabin wrote it into scores, making it the most performed color mapping in music history.