My letter-color synesthesia continues to fire on all cylinders well into adulthood. It's just a form of memory association of course. I can see that this 'e' is (in my editor) white text on a muted grey background. But that 'e' is also, simultaneously, the brightest lime green you ever laid eyes on. Like #00FF00, extremely light green. Every 'e' in every word lights up in my mind's eye in exactly this color. Most vowels do the same, consonants are inconsistent, numbers are pretty strong.

It's a fun little memory trick, and it was surprising to learn that it's not a universal experience. Not just "your letter-color association is different from mine" (it seems to be highly personal) but "wait, you don't do that at all?"