Orthochromatic photographic emulsion is sensitive to blue and green wavelengths, but not red ("ordinary" emulsions are sensitive to blue only, the natural sensitivity of silver halides); panchromatic emulsions are sensitive to red light also. IIRC, panchro emulsions were introduced before the war, but ortho film was still available until the '50s, and panchro a bit special, so use of ortho is possible. The giveaway is that red turns out very dark in photos made with ortho film, and looking at the roundels, that is possible. Blue sky tends to be too light, so for outdoor photography, it would be normal to use a yellow filter, which might affect things further. I don't know how lack of red sensitivity would affect the reproduction of exhaust stains, but I'm sure it wouldn't turn normal green/brown camouflage into a uniform white. HTH