I think the present color is semi-permanent, but it’s bound to fade, at least. Plus, her hair was recently cut, so it’s going to grow out within ... some reasonable period.
It’s perfect for the Tonks costume :-).
I admit I’m a bit shocked, I’m sure I’ll get used to it.
The nice thing is, in later years, she’ll tell her own kids what she did with her hair “one Hallowe’en,” and her children will be awed to realize their grandma actually ALLOWED it! What an awesome grandma! (Future tense, of course!)