This mask is paper with an image of a metal mask printed on it. I think the patina you see is perhaps the natural oxidization of the metal of the original mask. Copper has a green patina, so I think the metal mask must have some copper in it even though it is not copper colored.
Brass will patina as well. Wow, this is pretty good papercraft if I couldn't tell it wasn't metal! (I've not watched the movie to know if the design was yours or not)