I put together this sketchy tutorial on me lunch break - hope it's useful. Also, the light changed during painting (big storm) so apologies for the pics. So... Non-Metallic Metal.
Painting the Steel
For the steel you'll need these colours or similar..
Steel stage 1 - basecoat grey
Steel stage 2 - wash nuln oil
Steel stage 3 - paint the original grey back on around in the direction of where your light source is coming from... I tend to use two o'clock on the clock face…
Steel stage 4 - and the light changed! Add white to your grey and and paint a bar of light to separate the light and dark areas
Painting the Gold and Finishing up
For the gold you'll need these colours or similar…
Gold stage 1 - paint brown.
Gold stage 2 - agrax earthshade wash.
Gold stage 3 - add yellow to brown and highlight in same position as grey, around two o'clock..
Gold stage 4 - add more yellow and paint a bar to separate light and dark areas..
Gold and steel final stage - edge highlight with 2 or 3 very thin layers of pure white. Also firm up your horizontal light bars with this mix - et voila!