The best metallics on the market are Vallejo Metal color, or so say most of the professional painters I've ever seen.
Expensive, but a widely praised product.
I've used Reaper metallics extensively. I find them superior to GW paints, especially the metallics.
I would prime with black. Paint copper and leave as much of the black in recesses as possible. Wash it with a black wash if you need to mute it. A brown wash would be interesting. Highlight from there as desired.
If you really want to bring out the panel lines, use black oil paint to Pin Line. TAMIYA makes a custom bottle with a tiny brush in the cap. That an some mineral spirits and some Qtips and your lines will jump off the mech.
With paint schemes that are a single color, picking some other colors that break up the uniformity is important. Barrels, cockpit, lights, etc. Get some other color in there that makes it interesting. If you want break with the Canon and add an accent. A color compliment in a small amount will be interesting. Copper is a warm color. Choose a cool accent. (Green
, Blue, purple) if you are desaturating the color (adding a neutral) make the accent towards the pure color. Unless you want the whole mech to look washed out and understate the accents.
You could prime with a black/green and bring some of that out. (Again leaving the cool shadows)
