What color is her hair naturally? Chalk I think comes out really easily. There are also lots of colors of hair extensions that you can clip in. She could mix and match, not do anything to her actual hair at all.
Well, that depends on how you want them to look. If you want them all throughout the hair, I'd still pull a little of the crown up so they could blend in underneath and not be "on top" because when that fades or grows out it's less aesthetic. But totally up to you, you could alternate it on top of her head to give very dramatic streaks. I would look it up on youtube for specific tutorials so you can get an idea of how you want it to look, visuals are so much better than text!!! Foil and a comb.