Metal Cote >> darker washing (brown for golden/brass tones, black for Gun Metal etc.) >> Highlighting with the base color + a bit of Chrome/Aluminium...
Simple as that.
Heres an actual example - the shoulder pads from the "Imperial Knight":
This is just GunMetal as it comes from the glass...

and this is the other one - with some black washing and 50/50 Gun Metal/Chrome Silver

BTW:
If you want to create rugged/cut/destroyed areas, its pretty helpful to use Maskol here and there. Theres many methods, I found out that (for me) a sponge + Maskol works very good. Just cover the desired areas with some Maskol - careful, careful... - let it dry and then paint your color over it. Once its dry, you can peel off the Maskol (with a soft toothpick or brush)...
Tada!