Personally I don't see how any public school should be able to really say anything about the appearance of those who go there, as attendance is mandatory. After all it is a government entity, I mean it isn't like having blue hair is going to make the IRS not want it's cut, is it?
Private institutions can and should be able to do whatever they want when it comes to appearance standards, as you can just not go there if you don't want to. Of course, that is up to the parents and not the children but, at least there is some say in a private institution.
I have had hair that was past my shoulders since I was 12, and for the majority of my high school career it was near my waist, and I also wore eyeliner, mascara, and foundation (in a gothic more than feminine look) and the only thing they ever said was that I couldn't wear my spikes for safety reasons, and I think I can deal with that.
Perhaps my public school experiences were nicer than most, then again I didn't have any parents telling me not to dress as I saw fit, even though if it weren't for the scorn of society I would likely have been wearing dresses to school. And make no mistake, teachers and students alike had problems with the way I looked, but despite the occasional threats of 'guys wearing makeup disrupts class' there was never any official action taken as I was within my rights according to the school rules. And that is how it should be.
As far as purple being a gang color, I would tell them to prove it. And while they are at it, they need to prove that red and blue are not if being a gang color is now a dis-qualifier to the dress code. The truth of the matter is, that every major color is used for a gang somewhere, therefore it is nonsensical to be banning colors outright. Banning bandannas hanging out of peoples pockets, etc seems alright, but banning a color outright is just going too far.