I've done a cake with a similar dirty look to it! I personally used Cocoa colored petal dust and just dusted it on VERY lightly, it came out perfectly!
Thanks! Do you think this could be done on a buttercream cake also?
ACakechic87, I made this cake for my daughter's baby shower and I used dried cocoa powder and lightly brushed it on. It works like a charm! Dip your dry brush in the cocoa and tap off the excess so you don't overdo it. Not sure about this techique on buttercream but if it a crusting buttercream I would imagine it would work. Good luck!
Thank you so much for your help!! The cake is adorable