You know what I think? Prior to painting, the artist cleaned the side of the aircraft. Not many pilots did and what we're seeing is the cleaned aluminum panelling. That makes a lot more sense than everything else. And the deacon name - I've seen a couple of P-51D aircraft where that panel is lighter than the rest.
And lets face it, most of these planes were working in pretty grungy conditions - it's not like they had a aircraft wash station setup to give their birds a wash-down after a long week of flying in rain, taxiing in mud, taking off on grass strips etc. Some pilots however did keep clean birds, just not this one.
It's the only logical explanation. The pilots were working in a dirty environment