If you have a silver colored medallion, you can make it bronze by first painting it with clearcoat. Not sure it makes any difference what kind; Testors, Poly, Fullerplast, etc. Then go down to your local Trophy shop and have them dip it in the gold solutions that they use on trophies.
I know that works silver to bronze but I don't think they can make silver coin out of bronze. I believe that trophies come only in 2nd place Silver, and they make the 1st place Gold and the 3d place bronze by dipping the Silver trophy in right solution to turn the color.