I had previously painted the drum and chains black, so my first step was to tear up some of the foam used in the FoW blisters, dip one of the torn parts in black and dab along the front of the boom arms (where debris and explosions would damage the paint). I then repeated this with Russian Uniform Green (Vallejo Model Color 70924) over the drum and chains - making sure the black on the booms was also nice and patchy.
This gave a suitably mottled appearance - the sections I couldn't get with the sponge I used a brush on, which also worked fine.
I then repeated the process with a small amount of Hull Red (Vallejo Model Color 70985) over the bottom of the boom arms and lightly over the chains.
Next up came some Model Air Metallic Black (Vallejo 71073) which I had picked up by accident years ago. Then some Game Color Gunmetal Metal (Vallejo 72054). All applied using the sponge method.