Maybe just some washes to glaze the minis a bit closer to their faction colors. Then maybe a really light dry brush with a bright metallic.
From what I have seen of them, that's probably what I would have ended up doing.
Maybe not a bright metalic... I'm thinking of some rust patches... of course, I have no idea how to do rust or oil stains or any of it. But I like the idea of the colour wash! I was thinking patches of colour but a wash would be better all around.