When I was learning how to modify casts my mentor would always have me use colored plaster for any build-ups. Using colored plaster is a great way to know and visually see what your are doing to a cast. I know this is one of the oldest tricks in the book but it might be nice for someone to see.
The powdered chalk is found at your local hardware store by the area where the chalk line is. Chalk comes in a bunch of colors so have fun with what you use.