Actually I shouldn't post that here because the model is almost done. but IMHO there wasn't the black colour at the bottom and sides. Looking at the so-called colour images of the kite I would say the B-26 had the camo livery consited of the OD/NG. It is a very shabby in fact with many smudges. The D-Day id strips were overpainted or the white was washed off contrary to the black that wasn't that easy to remove. For the reason there wasn't the black spot around the dorsal turret but there was an "empty" area where the white was applied. For the same reason the "N" letter had the outline noticed at its top in order to make the code letter being better readable at the white background there. Using the gamma correction it can be noticed that the shadows get more irregular shape except the area where the bottom edge of the top D-Day strips was as it was the straight line. Also the black tone disappears at the fuselage sides especially behind the wing root at the trailing edge and in front of the leading edge. What is more a slightly enlarged shot of the fuselage front reveals pixels of different tone that make a wavy demercation line at the fuselage bottom. Additionally the image with the crew of the bomber also shows that the "demarcation line " for colour tones at the cockpit area wasn't the straight line but irregular one what what is a result of a poor quality scanning of an image printed on a shiny paper. So just a light trick again. Just my two cents.
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And here three B-26s of the 553rd BS ... none of them had the black colour at the sides and undersides.
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the pic source:
Martin B-26 Marauder in USAAF and USN service