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Rattle cans of the antique copper, antique brass, and antique gold. I did a base of the copper, a light middle layer of the brass, and a very light dusting of the gold. I did it against what you're supposed to do and sprayed the next layer when the first was tacky but not wet. This allowed for the next layer to mix into the previous one without sitting on top as a separate later. This technique created the effect of the metal color change present in the game.
Please note, you need to be very careful spraying the antique copper base. I would allow the black primer to dry completely and do a light wet sand, then let that dry completely again before starting the copper. Once you start the copper part, you have to be very alert of it so as to not create drips as it'll jack up future layers.