honestly, if you're using nihilakh oxide they should be tarnished, and the brass/bronze should be darker. It'll just look goofy if the oxide is on shiny models. And interestingly, apparently brass vs. bronze is passing out of historical usage because the contents are so inconsistent. So feel free to use a more bronzey color without offending the blood god. =)
When I did my last few daemon engines I was working from a similar idea. They would be mostly tarnished bronze with the high friction edges shiny, and oxidation accumulated in the crevices. All the leading edges are brighter, but I didn't go crazy with my highlighting. Using metallic paint means that it will catch the light pretty effectively already.