How long has it been going on? Is the whole tail pinker or is it blotchy? I've noticed this quite a few times in my golden axolotl (where his whole tail is suddenly noticeably pinker than the rest of him) and it's most likely just related to increased blood flow. Are it's gills also brighter? If so, it's probably that.
Either way, you should do some water tests just to be safe to check for high levels of ammonia/nitrites/nitrates.
A picture of your axolotl would also be helpful in order to make more accurate guesses.