I had girls ask about hair color yesterday as I have one who consistently dyes her hair green- dark at the ends and fades as you move up the head. It stays in FOREVER and when it starts to fade she gets it redone (she is 11). I have another little one with a hint of pink at the bottom of her ponytail, but it's already fading and I'm sure will be gone by our first meet, or at least faded enough to hide in a bun.
It's really not my preference, but it's also not where I want to spend my energy. So I just tell them as long as it's not obnoxiously wild it's fine. Pulling it into a bun or braid will cover up at least some of it. My warning to them was that while I was okay with it, I made no guarantees that HC would be. But we don't have a hair policy in place, so really no ground to stand on.