I am with JFJ in thinking that it is a color that is located under the top layer but I believe that it is just where the color washes have scratched off or didnt take to the paint for what ever reason.
Here is a pic comparing the exact same set of armor the only difference is that the pic on right has the black wash applied the one of the left doesnt.
I have also used the Panzer Olive green with good results, but put it on before the black wash.
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