It may just be a question of different levels of reflectivity between the NMF and the paint. Whether the more reflective material is reflecting a dark or light background to the camera.
No, it's more than that. There are images of metal with different reflectivity that are darker *AND* lighter (some are also identical), but they look nothing like this. Some of the darkest ones are with airplanes parked on white sand/coral so one might expect them to be lighter.
This is an applied or manipulated material. On St. Louis Blues you can actually see brush streaks. On Virginia Marie multiple sources say it is an applied treatment of zinc chromate that is also present on the nose. There are dozens of P-38s that show this same thing and it is distinct from metal differences, paint, reflectivity, or tarnish.


Those are paint or rag marks (brush/rag application or maybe an attempt at cleaning), not reflection.
So I think we have multiple possibilities for the variations we see.
1) Differences in material.
2) Factory paint.
3) Reflectivity.
4) Tarnish.
5) Touch-up paint, primer, or zinc chromate.
6) Wear, grime, grease, oil, dirt, etc.
7) Some combination of the above.