From what I have learnt the exhaust collector ring was made from some type of high temperature alloy that was originally almost pewter in colour but would discolour and take on a copper coloured hue after being heated up. The silver coloured ring at the front of the collector ring wasn't part of the collector ring itself but some kind of a shield, on early Hercules engines you will noticed that it has been spaced ever so slightly away from the collector ring while on later engines it seems to be flush with the collector ring. I think this shield was installed to protect the collector rings leading edge as they were expensive and difficult to manufacture. At some point during the war all night bombers were ordered to have all reflective or shiny parts such as the undercarriage painted over black, this was a problem with the collector rings because the heat would take the paint off, I think a special heat resistant black paint was finally used.
If you look at bits of recovered crashed Sterling you will notice that the Hercules collector ring hasn't actually rusted but has kind of gone a funny corroded white colour with perhaps rust showing through it. If you look at earlier Bristol engines fitted to museum aircraft like the Swordfish, Gladiator or Blenheim they have mostly been polished into a shiny pewter colour.
As a result of all this it is very difficult to know how best to paint these collector rings, in the old pictures the rings appear to be copper but were not, but neither were they rusty, they just never seem to look quite right when modelled. I hope that wasn't to boring.