The first thing to remember is that your metallic colors are not fundamentally different from the other colors that you have. You can still use all the same techniques that you normally do- you can layer them and highlight them up with lighter colors, you can add washes and shading with inks and darker colors.
The only big difference is that you need to keep your metallic pallet away from your other colors. You can shade, glaze and wash metallic metals with non-metallic colors, but you really can’t add non-metallic highlighting. In your metallic pallet, Mithril Silver is your new white.
I generally prefer to approach metals by layering them. The most standard gold color I use starts out with a dark bronze color like Tin Bitz or Molten Bronze with some brown ink mixed in.
From there, it is best to layer up by mixing a medium gold or brass color in (like P3 Rhulic Gold or Citadel Shining Gold).
You can add in final highlights using Mithril Silver. The more you add to it, the lighter your golds will be, and they will begin to look more worn to an extent.
Now, I just gave you a strong recipe for gold. Don’t let that hold you back. It is easy to find just one approach to painting gold metals and use it over and over again, but if you try to mix it up some, you’ll find that there’s a lot you can do.
For example, what happens if you take your dark metallic metal and mix it with your Mithril Silver? What happens if you highlight your metals with Boltgun? What if you use a blue wash over it rather than a standard brown one?
From that combination, you get some interesting metals, like the ones on this Mule: