Thank you for replying mate.
I have no idea if this plane was delivered in Natural Metal and subsequently painted, or was delivered already painted in the Olive Drab/Neutral Grey scheme (in this case, why they should have changed the colors of the camouflage? ).
This that you write confirms my opinion that is not entirely impossible that this plane has been painted in Dark Green + Silver (or better Natural Metal). In fact I had found something like that in "Camouflage & Markings" #16 dedicated to the P-51 from which I share two extracts that further confirm this conclusion:
Unfortunately I couldn't find more precise information on the P-51B WR*Z, but at this point I think I don't make a serious mistake if I use the Dark Green/Natural Metal scheme.
Thanks again for your contribution.