Duke Ellington described his colored hearing in terms musicians still quote: a note was not one color but a color per player, dark blue burlap from one saxophonist where another’s same note was satin. The account, preserved in the memoir literature around him, ties color to timbre and to the individual voice of each band member.
That detail makes his case unusually interesting. Ellington famously composed for specific players rather than instruments, and his synesthesia, as he described it, perceived exactly that: the man, not the part.
As a pre-testing historical figure his evidence is his own recorded words rather than any battery, but the words are firsthand, specific, and of a piece with how he demonstrably worked.