It should be in the rule book that you can punch anyone who has their number bleached in their hair. It's one thing if you go Anthony Mason and get a bunch of shit cut in your hair and it's another to bleach stuff.
I'm okay with the little blonde streak in the front of Dennis Schroder's hair. The number in the back is too much and Isaiah Thomas had every right to smack him upside the head.
You're a grown ass man. You don't bleach numbers in your head. If you do you get punched.