AI'm going to be making a zombie cake this weekend. In gonna carve the head from RKT and cover with modeling chocolate and then fondant. I want to achieve a dirty type look to the zombie , kind if like a chalk dusty look . I was wondering what I can use to make him look dirty and dusty

Try dusting him with powdered food coloring... a little goes a long way.  Cocoa powder seems like it also could work.

By the way, I just noticed your user name.  My last name is Rocco.  Too funny!