I have an adopted Korean daughter who we have had since she was 5 1/2 months old. She first requested blonde highlights as a first grader. I felt she wanted to fit in with her adopted family (I am blonde, so is her brother, father is brown); and to better fit in with her mostly caucasian class mates. Even though I was not thrilled with the idea, I allowed it because I felt it was just hair, not worth a battle, and maybe she would feel better about herself. Everyone loved it, after awhile she decided she did not like them (a year later), and we rehighlighted with a dark red that blended better. She rehighlighted one more time, but now has gone back to her own color recognizing on her own that her hair color does nothing to change how people like her or perceive her.
I would suggest not making this a huge deal. Facilitate conversation between yourself and the birth parents so that all of you are supporting the daughter in whatever challenges she is encoutering now. The struggles once faced in middle school now hit in elementary school and we parents need to recognize this. Support her!