Scheme 1
Lay a base coat of flat black. Mask of the letters/designs and spray those in a bright orange, yellow, lime green etc. Mask off the letters again, and spray the cannon in a corresponding bright color (not the same as letters/design obviously). Leave the flash supressor in the flat black.
Scheme 2
Lay the same base coat, but spray the letters in a metallic silver. Re-mask letters and spray cannnon in a midnight blue metallic and lay a final coat of Candy Purple. Leave flash supressor flat black.
[NOTE] I have sprayed this on scrap PVC and it looks totally awesome!
Scheme 3
Use chameleon paint and paint the whole cannon the same color. As cool as this would be, it would suck to get a scratch...Can't do touch-up paint with the chameleon paint

Those are my ideas thus far. Feel free to tell me what you think or if you have any other ideas. Input is greatly appreciated!