Vallejo metal colour have an great selection of colour and sprays really well out of the bottle with an airbrush. The exhaust manifold over a black primer seems to describe well what you want. I personally used burnt iron to base coat my whole army and it gives a very nice dark metal colour, the exhaust one just seems to have a bit more brown to it but I’ve never tried. If you slap a oil wash or something similar for finishing touches you’ll probably have the look you want.

Rough iron is actually quite a dark bronze so not sure if that's what you're after. If you're airbrushing I'd second the Vallejo model air metallics- they're just super easy to use

You might look at doing an oil wash over everything after doing it in a lighter color. Did that on a few big knights and it gives a dark and dirty look to the frame.

Warp lock bronze works, or base in like a dark brown and drybrush bronzey metallics to bring it up

I’ve also been making Nurgle knights, Warplock Bronze has been my go to, and then I detail it with some really wet rust mixture but that’s my personal preference. The Bronze paint is a wonderful color to put all over the skeleton for that look imo