The P51 wheel bays were, like the inside faces of the undercarriage doors, bare aluminium, with a protective coat of clear varnish. The front face of the main spar, which formed the rear wall of the wheel bay, was Zinc Chromate, Yellow.
Those pics showing green, Interior Green or Grey bays, are normally of restored aircraft, where the owner, quite naturally, wants some form of protective coating on his valuable bird.