Do you mean darkened copper or greened? Verdigris is the term, and you can produce it in several ways. Just keep in mind it is a bit poisonous and will smell on your hands, so it should be laquered. This will change the color also. Just google a bit and you will find several ways to create it.
You can also use solutions and electric currents to add a copper finish to aluminum dice. Then you can age this patina. It looks outstanding if done correctly.
Nice to make simple tryouts, a bit much to do in a large scale :D
Dev