I often find myself wondering about this. It came up again recently as I was sitting on a bench in a park in Boston, Massachusetts. A passer-by stopped and sat on the bench, and after a few minutes of sitting there in polite silence, she turned to me to speak. At that point, I noticed she was wearing
opaque glasses, which I took to mean she was blind. She asked if she might ask me a question. I replied that she certainly could. This was her question:
"As you can probably see (and if not I apologize!) I'm blind. I have been asking this question to people off and on for years. Do you have any non-visual sensations that you could equate to colors?"
This floored me. It took maybe ten seconds of "Err...um...lemme see...well..." before my brain kicked into gear, and I replied "Oh yeah! I got two off the top of my head." So I gave them to her. Here they are:
She sat there for a moment, 'looking' off into the distance, and then said "Thank you." At which point, she brought out a small computer and typed something into it before smiling, wishing me a good day, and moving on.
My fantasy is that she is going through life collecting metaphors for colors. I hope she is. I hope she one day publishes it, whether in poem, prose, song or painting (or any other medium). In retrospect, it seemed such a wonderful thing to ask of many many complete strangers. I often wonder if there is a convergence in the answers for particular colors, both in the sense of similar events/sensations and in the way that her mind 'feels' the examples.
After that, I found myself wishing I'd asked her about synaesthesia.