Went to a concert of Messiaen music, which is quite a challenge to listen to. I closed my eyes and watched the color-play. Then learned that Messiaen was synesthetic and experienced music and sound in color too!
It’s almost impossible to visually represent the experience, though … The ‘colors’ themselves are also perceived (by me, anyway) as mouth sensations, shapes, textures, ‘scenes’ or place-scapes, and emotional states. And the context informs the color.
So it’s not like “B flat is green” - it’s more, “This B flat in this series of chords is a daub of grayish green against a wash of shifting blues and browns, with a round sensation on the front of the tongue, poignant, pretty but sad, a painting of a Dutch harbor town in early Spring, seen in a chilly museum on a Tuesday afternoon.”