Dyeing black/brown hair a dark green?

I'm cosplaying a character that has dark green hair. Not vivid green, but something dark and somewhat subtle. Something that doesn't shine obnoxiously like stainless steel in the sunlight. In the process, I don't want it to be permanent. My hair is a black/dark brown color. It has to be temporary, washable, or revertible in a simple process. What could I do to accomplish this?