Honestly, he might just be marbling. From your pictures it doesn't look like anything's wrong-- I can't see any tears or weirdness on his fins and his scales look fine. You mentioned he's perky and his behavior hasn't changed, so that's also a good sign.
Marbling is perfectly harmless and not something you need to worry about. Sometimes your fish will change colour! :) It happens. As long as his behavior doesn't change and you don't start to notice anything that looks like actual fin/scale damage, I'd say he's fine.