Yep, I would agree, browns (like matty said and also in the example given by Nigel). The key to it though I think is you need to highlight under the scratch/ damage with a lighter colour than the armour, so in this case pure skull white. This will help give it depth, and create the illusion that a small chunk has been chipped out.
This may be a problem as the pictures given looks like the armour is already pure white.
Would reccomend you do it on a test piece first too!