As Silver suggested, a brighter green on the goop inside the tank, and then after you've sealed it, give it a coat of 'Ard Coat, which will make it shiny.
You can paint your metals a bestial brown, then sponge bolt gun metal over it. Mix up some blazing orange and vermin brown and splotch it over the metal for rust ... then sponge over with more boltgun metal.
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