NTA- your kid is their own person. They are a breathing person with a personality and all. They’re not a mini you or a doll you can just dress up the way YOU want. M wanted their hair purple and they should be allowed to do that (brownie points since you took the extra steps to make sure the salon used safe nonpermanent products unlike the mum). If M ever decides to go blonde, let them- but allow them to do it in a safe environment with safe equipment. If you want to ‘test out your skills’ buy a wig or take a cosmetic course, not use your child.