Have you tried an eraser? One of the more artsy variety, either art gum, or that other one. Drat can't remember the name. One is grey I think and more like chewing gum, the other is about the color of a Qualatex Mocha balloon, maybe a little lighter. It reminds me of the soles on some of my dress shoes. Also try the website
www.clown-shoes.com They have information about taking care of your shoes.