Besides shoe polish, is there anything that can be used to add color to leather that has be scratched or worn down?

Go to the leatherique website, they have exactly what your looking for. It's used my many many MB owners, including myself. Used propely will make the leather look and feel like new. The basic kit cleans and restores, and they have fillers for damaged areas along with dies to make it look like new.

http://www.leatherique.com/