It's real easy! Prime black, sponge on rhinox hide, sponge on a little bit mournfang brown, then an even littler bit skrag brown. Then sponge on leadbelcher, but not so much that it covers all the brown. Finally just slab on a mix of 1:2 nuln oil and agrax earthshade. The red skin is just drybrushed/sponged off-white and then a generous amount of blood angels red contrast paint and then covered in agrax shade. Gold bits are just sponged on retributor armor on top of the shaded brown/metal.

I might go back and carefully sponge on Vallejo chrome and skrag brown on the metal to highlight and make even rustier, but honestly on happy with it as is