My girl is not of that age now (she's 4 and a half), but I don't have any problem with green hair either. I will encourage her to do what she want to do, not what the other want her to do. Of course, there is a limit that I wouldn't want her to pass. I wouldn't accept anything that would affect her health (tongue piercing, cigarettes, etc.) I spend much energy now on her health, I would find it discouraging that she damage it by purpose.

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>Actually, we did green hair. #2 son had long blonde hair dyed a pretty shade of pine green with semi-permanent stuff. I encouraged it because he had a great need to do outrageous things and green hair is pretty outrageous, and more importantly, NOT PERMANENT.
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>Toungue piercing is disgusting. That's why I encourage the non-permanent stuff to satisfy the need to be outrageous. Now #2 is "preppy" which is quite conventional.