Mine came with a plastic stock. The trick to that is dabbing a little bit of acrylic paint on a cloth and "dry brushing" it on. It leaves some of the deeper areas black, for the grain look, while coloring the rest brown, or whatever shade you choose. I used two different browns. People are amazed when they knock on the stock to find out it's hollow plastic.
For the metal parts, ( mine are resin) I used gunmetal spray paint from a hobby shop. I've also found that at I can get the same color if I paint the gun a silver color, or aluminum, then before it dries, mist it with flat black. It takes a few minutes to get the hang of it, but you will end up peppering the silver with black evenly enough that it looks gun metal.