Honestly how I see it is. Anna's hair turns white and in my opinion, they should of left it like that to kinda symbolize a scar. Like in Howl's moving castle (spoiler), sophie at the end keeps her grey hair from being old because its like a scar its to symbolize one. Just my opinion, but it looks really good.
And if you like white-haired Anna, you should see this pic: kradeiz.tumblr.com/image/89986… I didn't draw it, just reblogged it. But it seems to be an AU idea where Anna was given snow powers instead of Elsa.