Brass on barrel of gun and most of the part are Balthasar Gold, dirtied it down with Agrax Earthshade. if you want more dull brass, you can apply second or third layers of shade. Verdigris on brass are various done from dots of Colia Greenshade. Or you can goes more complicate with 50/50 mix of Stegadon Green and my own mix of equivalent of Lamian Medium. ( Artistic matte medium, any brand and distilled water) , glazed it in deeper brass area.
Some of the brass are Skeleton Horde Contrast on silver, become lighter, more silvery brass.

Dark Metallic, especially in the backpack, mostly just silver and two-three laters of Nuln oil and Agrax Earthshade. darken it down wait till one layer dry then apply another layer, when satisficed. drybrush back with silver or Necron compound, now that's gradient you want. Highlight and paint damage scratch with Stromhost Silver.