Check
Buypainteds IMperial fists painting guides. He does it with airbrush and these primary colours: Radome Tan, Gold Yellow and Burnt Umber I think for highlighting and shading the gold yellow. It's over Vallejo grey primer.
These are the scouts he did, so they come out quite dark. Not sure if they were done the same way as the rhino vid I linked above, but should be easy enough to find.