To add extra interest to metallic paint you can add one or two glazes of normal colours. This increases the depth and also helps to bring the metal part together with the rest of the mini. I'll create another tutorial at some point on how to apply glazes but for this one it's simply the idea that adding a glaze can have a dramatic affect.
For one glaze you can use any colour (although to shade it more you may need to increase the contrast with black added into the mix). But for two colours of glaze it works best if you use complementary colours (red green / blue orange etc...). This is due to the complementary colours tending to black when mixed (although in practice it often goes a brown black).
For the butcher on the steel metallics I'm using the following colours -
These give the following results
On the G.A.St.Art cow I followed the same principle with purple (and green by the looks of it, although I can't remember) -