@Harry
I did see your work on Britmideller, you’re getting the hang of the airbrush with admirable aplomb.
As for the tonal variation, there is usually a yellowish anti-corrosion etch coat applied to airframes before they’re painted, it’s entirely appropriate for various parts of the airframe to show variation as parts are often recycled from retired/stores aircraft.
Its good to experiment with undercoat colours to bring out the top coat – I undercoat yellow bands/wingtips in white first and if I’m painting an aluminium airframe, I’ll selectively pick panels to undercoat in contrasting blue, black, white or grey. You don’t have to be super-accurate, as you then pick out the panel lines in black before airbrushing the top coats. I’ve just finished a 1/48 Hunter with an aluminium underside done this way.