Annex14
deposits on IPT fragment
I think the bright shiny to grey color comes naturally over time. This is a hot area of the engine and the combination of temperature and air turns things grey.
The darker coloration and lighter irregular deposit on the surface close to the bore probably came from both heat (darker color) and liquid metal (lighter color) migrating out and solidifying on the disc surface.
As you point out, the rim of the disc (under the dovetail slots) looks remarkably clean. For this section of the disc, there was no apparent metal contact of the disc rim with another component.
I think the metal "ring" that you see hanging in the engine is casing material, some is totally gone, some is bent aft or forward. The in between space is where the disc, blades and nozzles exited the engine. There are two pipe rings that run circumferential around the outside of the casing at a location directly above the IPT rotor blades. I think these pipe rings provide air and are for blade clearance control, mainly at cruise altitude. You can see in the photographed area, they are mainly gone except for a piece of one still showing.