Step 21: Painting
The color scheme is fairly simple, black, gray, and dirty white. The bullets are gray and yellow. That's that.
What, you want more? Well, okay.
The entire barrel and handle assembly should be black. The back half or so of the ammo barrel and spent cartridge chute should also be black.
The front half of the ammo barrel should be dirty white and the fastening point for the ammo barrel's supporting pipe should be gray. The pipe itself should be black.
Other than that the only other advice I have is that with this much plastic using some sort of paint designed specifically to adhere to plastic is a good idea. It will make your life much simpler and will make the paint job much nicer looking.
After the rough painting is done a few specific details may be added, if you feel so inclined. There is a black line (probably a metal seam) that runs along the side of the ammo barrel with five large headed steel screws above it.
While this bit of detail is unecessary it can be useful to add to the illusion of a heavy come to life.