Probably discussed before but probably lost as well. Many people believe her hair is gradient. Logically, many artists draw her this way as well.
I personally disagree though. Probably others said this as well, the colour of her hair is caused by the scroll above her head. Especially since the scroll is emitting a strong purple glow. However Anthony on IRC mentioned:
?21:35:03? <Anthony> if it was that
?21:35:12? <Anthony> wouldn't the rest of her hair be multicolored as well?
?21:35:29? <Kilgamyon> No, because it's not as close to the scroll.
A good comment our Reisen Lover (Kilga) makes. We simply don't know what exactly is going on. So let's take a look at ZUN's official work.


Now there is another issue. Let's say the theory of the scroll going glowy causes this. So why is her sprite still showing the purple gradient hair when she folds her scroll? It wouldn't make sense no? Well, I personally think that would cause too much trouble if ZUN decided to change her hair colour. Because almost nobody pays enough attention in the heat of the battle to the sprite animation. Not to mention spriting is a pain in the ass anyway.
I wonder if this question was asked during the UFO interview on youtube. As far as I know they did not discuss her hair at all. Actually, Byakuren was briefly discussed in that movie.