Metal

Thats a perfect start. Normally what I try to do, is when I get a render, I try to find a colour in the render thats used alot in it, but not at the edges, and hopefully its a dark colour. I use that for my backround, Then I pick 2-3 more colours fomr the render, or that would fit with the render then I start layouring out grunge brushes 1 brush per layer, wihle switching colours. Then I can add the redner in, scale it, and move around all the layers (hence the "1 brush per layer" part) if it doesn't look good.