Given that I manufacturer NMF paints including copper I have found over the years that if you use a little bronze, black, gray, green you can create the illusion of the aged patina that copper takes on when exposed to the elements. It is just a matter of highlighting and blending the colors...experimentation.
I once did the model of the Vietnam Vets (I can't remember whos kit it was) that after I had assembled, filled it with sand and plaster, then paint with a blend of copper, bronze, gold, black turned out to look and feel like a real metal casting. It sits in a museum and last time I saw it still looked as good as the day I finished it...only the natural patina has added to its look.