My sister starting in HS started dying her hair. That was before the real wild colors were around, but she did various shades of blonde, auburn (reddish) and also black hair just because. She also cut her hair. So while what she did with her hair was still acceptable job-wise, it was a way to express her mood and style, which was usually edgy. I still remember a time she came to visit me. the last time I saw her her hair was blonde, and shortish. At baggage claim I walk by a woman with almost shoulder length jet black hair wearing mirror aviators, and think to myself, well that person's not from around here. And then the person called my name which totally confused me and I realized it was my sister.
My oldest daughter when in middle school, had a sleep over, and during the sleepover dyed the bottom half a girls hair with koolaid powder, after bleaching it. All done in the bathroom. Which started a cascade of girls in that group dying their hair various colors. When my daughter was getting her haircut, she requested a stripe of color (she decided on red). To me I don't think it had any deeper meaning than experimenting with how one looks. Definitely as a parent, MUCH prefer experimenting with hair, than things that are permanent like tattoos, piercings.