Question

Mum and dad are very angry with my brother. he (his/hair/dye) bright red last weekend.​

Answer 1

Your brother has the right to look the way he wants, they shouldn’t be mad at him.
It is understandable if he’s a minor though as parents have most of the rights about what the child could do but still it’s just hair, if they really hate it they could make him dye it back to his natural hair color.
Bright red is a bit extreme so I do see where they are coming from though.