I saw a demonstration by a blacksmith from the UK once (unfortunately, I can't recall the name, sorry).
He had some sandblasted pieces of mild steel sheet, and what he did was put them in the middle of several types of wood shavings (in a tupperware box). He added some water by fine-sprinkling it, and after a couple of minutes, some sort of acid in the wood shavings had reacted with the metal, giving it really, really nice colours and patterns.
I can't remember the types of wood, but I'm guessing wood shavings aren't that hard to find/make - I'd try oak, but loads of others ought to do the job. You can also use different types of shaving to give different patterns. It was amazing! Maybe leaves work too?
Never done it myself, but I've seen it with my very own eyes!
Just make sure the pieces are clean. Not polished, just clean!