My vote goes to purple. Both are really cool colors, but the darkness of the purple makes a nice blend with the black I'd say. The green doesn't necessarily make a bad blend with the brown (if you choose to do that), but it isn't as good as purple and black.
From an artistic perspective, black goes with everything pretty much-- I mean, black is basically a combination of all colors. (You can also substitute black for doing shadows when one paints, but that is another story lol). Green and brown can be iffy though. It all really depends on the shade of brown and stuff. Sometimes they don't really go together.