I'm not going to marry him if he decides to dye his hair.
Why would his hair stop you from marrying him?
He's the hottest guy I know, I just don't want to marry him if he is going to dye his hair into a stupid color for the wedding
If he is happy just let him dye his hair, you shouldn't cancel the marriage for something this trivial. Better yet, why don't you support him by picking out a hair dye that you and him can both accept?
It's all about his appearance. He doesn't know how to act and just wants to be all flamboyant.
It sounds like you are having second thoughts about who you are marrying. If you don't approve of his behavior or respect his choices about something as trivial as hair color, how will you support him over something major? It may be worth talking to a relationship counselor.