Hey M.!
Well, my son doesn't color on walls, but I used to. I was notorious for it!! Used to drive my mother crazy. One day she decided that I could have one wall in my own room to color on. I was only 5, but it was the coolest thing ever!! I still remember it. Maybe you could give that a shot. I turned out all right and now I get paid to color on other people's walls. I paint murals in kid's rooms, so I guess it paid off. If you'd rather her not do it though. Try those new crayola paints. They only color in crayola special paper and it's huge paper too.
I really have to suggest the one wall thing in her room though. It will be her own artwork and let her be creative all at once. After it gets crazy, you two can wash it off and she can start all over again!!
Hope this helps!!
Good luck with your little artist!