I would tell her to go back to natural (or close--whatever works), and I would have her pay for it. She knew processional was a possibility, and this is not a new rule. I understand the teacher saying "no" to a wig--sure, you can get a Halloween wig for cheap, but then it would look...cheap. If the teacher allows one wig, she'd have to allow them all.
As to why I'd make her pay for it? Because she's almost an adult, and this is something she could have predicted. Having to change her hair color, and pay for it, is an important life lesson. Sometimes adults have to do difficult things they don't like to do. Kind of like, if she got a speeding ticket, you wouldn't pay that off for her, would you?
That said, I might, after the fact, give her extra "processional" spending money, maybe for a birthday or Christmas or something. It's not the same as paying for it, but it's a way of acknowledging that she made a difficult choice.