From an interesting article I found today:
Long dismissed as a product of overactive imaginations or a sign of mental illness, synesthesia has grudgingly come to be accepted by scientists in recent years as an actual phenomenon with a real neurological basis. Some researchers now believe it may yield valuable clues to how the brain is organized and how perception works."The study of synesthesia [has] encouraged people to rethink historical ideas that synesthesia was abnormal and an aberration," says Amy Ione, director of the Diatrope Institute, a California-based group interested in the arts and sciences.
The cause remains a mystery, however.
My piano teacher said he could see colors when he heard music and that certain keys, like the "key of C", had a certain color.