Global Scale Modeller

Same as the hull? Black? Steel? Fully rusted?

All the variants you mentioned depending on the historic background of the vehicle if we talk about German ww2 armor. not sure how it is handled in other cases.

My thoughts only...no proper knowledge of this.
heavy steel rope today is normally galvanized with a oily coating, this becomes dry...ish grey then progressively rusty over a period of years. Rust is normally a light reddish brown colour.
I don't think it would have changed much, perhaps later in the war the steel may not have been galvanized. Would still have been oiled though and it does take sometime for the oil to disappear from between the strands.