If it's any consolation I have a fantail that is predomintaely white, though some orange patches, and it developed reddish lines on it's tail. I was worried at first, but kept monitoring it. They seem to come and go a little, and appear to be a natural variation in the fish. I'm pretty sure comets change their colour a bit as they get older.. I guess it depends how old your fish is.
I would keep an eye on it, and see if any other symptoms develop that might give you a clue to what it is (or isn't as it may be) I would probably lay off the antibiotics for now, IME they should only be used as a last resort, and you've already used it so recently.