Hey Fencer, about the grey colors on the fuselage and the white G-2 nose:
maybe the white is just wearing away? The white color often was temporary for the winter season, and even if it wasn't the white distemper white-wash, it would still be a temporary paint of coat. The grey on the fuselage might be a bleed-through of the colors underneath.
Wild guess, but because the nose IS the first thing into the wind, maybe the G2's discoloration is an old paint color showing through as well?
For the most part, earlier in the war (from the beginning into the middle era) the 109s were well cleaned by ground crews. Compare photos of 109s to photos of 110s. 109s have very little exhaust staining (for the most part) but the lesser 110 squadrons have huge black smears stretching far across them, starting from the exhaust points. Same engines, just less preferential treatment by ground crews.
I'd think most basic gun residue stains were cleaned off after a sortie. A quick wipedown I imagine, but can only guess. FYI they do the same with F-16s today because the residue is highly corrosive and can damage the skin of the aircraft.