They deleted camouflage since it was no longer considered necessary,same for bombers late in the war. The weight penalty was minimal (about 30Kg for a B-17 IIRC) but it saved time in production and should have reduced drag slightly.
The Mustang wings were primed,puttied,sanded and finally sprayed with an aluminium laquer to give a smooth laminar type airflow over the wing. Spitfire wings,at least the front portion,were treated in a similar way though the retained a camouflage finish. There is anecdotal evidence from maintenance crews that at least some Mustangs were de-puttied and returned to natural metal wings in theatre but that is not how they were supposed to be and certainly not how they left the factory. I don't consider myself an expert on USAAF aircraft (by a long chalk!) but scanning the images I do have,I can't see much evidence for natural metal wings.
Cheers
Steve