Step 1: Paint the gun.
I used the cheap acrylic craft paints from Michael's (Americana or Folk Art) that you can find on sale for $0.39 each. For the main body of the gun (yellow and gray parts on the original) I used the Raw Sienna color. The recessed and raised details are Pure Gold. You will probably need two coats to get the right coverage. For the "metal" pieces I painted them Slate Grey first, then used my fingers to rub/buff on some Metallic Silver to get the right look.