Yup, colour change.
Plastic plants and ornaments with sharp edges, just enough to catch on a nylon stocking, can tear a fin, so check those with an old stocking - if it catches, take it out.
I can't see rot (I have recent experience with it, sadly, and this doesn't look like it) so maybe he's tail biting. I've read that tail biting is sometimes caused by boredom or stress - does your betta have a cave or soft bushy plants to hide in? Fake ones are ok, if they're silk. If he's got a lot to explore, he might make himself too busy to bite it.
Looks to me like there's a little regrowth happening already? Or is it just my tired eyes?