The black, and brown, colors of aging can be accomplished with gun foulings, as mentioned, or diluted brass black. To accomplish the light greening most old brass fittings have around the edges, a solution of vinegar, and table salt, can be brushed around the edges of the fittings and screws. It usually takes a day, or two, for the green to start appearing. It will be a light turquoise when dry, and a deep green if oiled or waxed.
I did a wall display for a saloon years ago that was comprised of all modern replica firearms, and related equipment. All components of the display were artificially aged, and battered, to fit the decor. They finally took the display down, because people kept stealing parts of it, thinking these were valuable antiques.
Hungry Horse