Natural blonde brown/black is the only acceptable color.
...no but seriously, I really can't care at all about this. I find it less annoying than piercings or tattoos (I'd be interested in some sort of comparative poll of what unnatural needless body modifications people are the most annoyed by), but even those I can generally tolerate depending on how it's done.
I knew a guy who dyed his hair purple. Unusual for a Christian homeschool group, but he seemed fairly normal. I remember his brother as a douchebag though, so in that family hair dying was clearly a positive (then again maybe his brother was alright and I'm just judgmental...hmm...).