I saw a boy (maybe 10) with a bright green mohawk and my daughter said he told her that the dye is permanent. A friend said she wouldn't let her child get permanent hair coloring at that age, and particularly not something so dramatic. She thought kids should be teenaged before giving them so much freedom with hair.
I would be fine with it, personally. However, I also know someone who colored a toddler's hair because she thought her dd would be prettier blonde. I can't imagine having a toddler, (like even younger than 2) sit through and have chemicals on her head. But this toddler's grandmother was a hair stylist so she would have known what she was doing anyway. I don't know if that makes a difference on not.
My overall feeling is that if they're old enough to initiate the conversation and ask for it, then I'm OK with doing it/getting it done. Even something dramatic. But I'd want them to be old enough where they've formed an opinion and ask for it themselves.