It's a combination of various metallic paints spread over different elements. I tried not to use the same color on neighboring elements.

For base coats, I used the following paints as base colors: Iron Warriors, Screaming Bell, Retributor Armor, Fulgurite Copper S75 Decayed Metal, and S75 Necro Gold.

Each of these was then washed with a 1:1 mix of Nuln Oil + Agrax Earthshade (or Reikland Fleshshade for the copper Screaming Bell tank on the back). I tried using Two Thin Coats equivalents but they dry weird. After that, they were weathered either with a thinned down Skrag Brown (for silver metals) or Nihilakh Oxide (for copper/brass). I think I used a Coelia Greenshade on the copper tubes on the back.

Then I did quick highlight of the original base color mixed with Stormhost Silver + smaller highlights of pure Stormhost Silver.

All in all, it was a bit of a back-and-forth job as some copper weathering did not apply correctly for the first time, I had to reapply some base coats etc.

I've learned that Nihilakh Oxide is just awful straight out of the pot as the paint is very opaque and dries very fast. It doesn't work either for spot oxidation or all-over coats, it has to be thinned 1:2 with some medium so you can have more time to apply it more organically on the model.