When you apply a substance to create a certain type of oxidation on a metal, it is called a patina. The major supplier and information source for patinas I've seen is a company called Sculpt Nouveau. I think the color you want is the Tan patina listed here: http://www.sculptnouveau.com/Details...=34&category=6
However, you might want to search out an example of what that particular patina looks like on steel, because they each have different effects on different metals.
Again, you'll need to be making mild-steel or carbon-steel chain mail for this stuff to work. Less is more with chain-mail, since I don't know if you can do a decent job sealing (clearcoating) chainmail, which is what is normally done to halt further oxidation and coloring.