What I have done on mine is a permanent marker then I go over it with a lighter; this burns it a little and it blends in color wise and it seems to stay put longer. Kind of waste of time because I beat the crap out of my MP5-F clone and with have more dings,scrapes and other abuse marks.
This is more of an aesthetic thing on my part because I dont like the nicks and scrapes in my finish, but this is temporary until I send it off to Jayson at IGF for a refinish.
FYI, you are asking for trouble if you do this then sell the weapon. That will net you some negative feedback on Gunbroker I bet.