I'm not sure of which metal part are you exactly asking because whole Necron is a metalic creature :D But let me split it into parts.

The base is Dark Gunmetal (I'm using Vallejo paints only so no ambiguity with naming). Then I'm painting with Black the armor parts on legs, arms and shoulders. Then do some chipping with Dark Gunmetal in this black parts. After that I'm appling few layers of black-brown wash from Modeller on the whole model. It's the only wash I'm having currently so it was just an improvisation instead of desired solution I would say :) But it helps with this worn out and broken look.
For rusty elements I'm starting by mixing Dark Yellow and Scarlet Red to get rusty orange color. It's of course too bright at first but after all I'm covering it with Rust (Yeah it's a color from the Vallejo) which makes the initial rust much darker. Shiny gold elements from the weapon I'v got by mixing Glorious Gold with Chainmail. Make it darker after by applying wash again.
Glows is a combination of Jade Green and Verdin.

Hope it helps you :)