I saw nothing absolutely wrong with letting Willow cutting and dyeing her hair pink. Preteens and teenagers are going to experiment with fashion. I would rather have them experiment with fashion options than to experiment in more dangerous activities. Fashion is fun and a creative way of self-expression.
THIS is the TIME to freely and fully create and have fun with various fashion personas. Young people CAN do things with fashions that older people CANNOT. Let preteens and teenagers experiment with all aspects of age appriopriate fashion. This is the way preteens and teenagers learn about their fashion wants, desires, and personas.
Preteens and teenagers who are allowed to fully experiment with fashion will not rebel. They know that they can BE and DO freely. As they get older, they tend to make more mature fashion choices as they were allowed lots of leeway in their formative youth years. Their attitude is been there and done that, let's evolve into a more mature fashion persona. As a result of the experimentation, they learn what suits and does not suit them in terms of fashion. They also realize what was appropriate as a preteen and teenager is not appropriate as they become older.
Preteens and teenagers who had parents who did not allow them to experiement in terms of fashion will rebel. The girls whose parents did not allow them to wear make up and/or dye their hair once they become a certain age will literally go all out. Since they were not allowed to experiment in terms of fashion in the parental home, they go buck wild once they leave home. They also do not have the maturity and/or independence as far as wise fashion choices go. They are fashion infants so to speak.