Hi Jay .
Look at the wing of the bi-plane that just shows to the left of the DVIII photo ...this clearly shows a red star on a WHITE DISC ....showing that they HAD transitioned to a white field at this point , if & when the background was a dark ( non contrasting *) colour .....the DVIII probably being a later aquisition than the bi-plane in the main photo .
Also , ( remembering that it snows quite a bit in Russia , whitness the Skis on the Fokker)....
*****( 1 ).....You have not given any reason as to WHY they or anyone would think to paint (weight) the Underside of a wing to match the (summer) terrain , rather than the sky ....*****( assuming the machines were not stored inverted !

)
I have suggested some plausible reasons why the underside "appears " dark , but was in fact a light colour , hence the contrast of the red star against the background is sufficient in this case as not to warrant a white disc .
***** ( 2 ) ..However there is indeed , ample proof "in the same photo" that a white disc background to the red star , was currently employed on other aircraft if the need arose ( ie to contrast against a dark colour) .*****
Similar confusion has occured in a number of WWI German aircraft photos , where the fuselage photographed "dark" & therefore it had been assumed that they were painted/stained a dark colour .....however in many cases , the Mil' numbers have been almost undetectable in the photo , having been painted in black .....although in practice the numbers were easily seen against the "real" colour of orany/yellow/-ish of varnished plywood .
In cases (eg. RolDVIb etc) where a fuselage really has been camoflaged darker colours , then the serials have been painted in a contrasting colour such as white or light blue .
Jay , please explain your thoughts & answers as to the above statements (1) & (2) .starting / ending ***** ..............
PS * ..by "non contrasting colour" , we are talking about that which is detectable from a reasonable distance , where all colours tend to grey , ( the darkness of red & green being similar at distance )... for it is mainly the dark against lightness that provides the visual contrast ....Hence the need or not , for a White or light background .
Regards John M
PS ....Check out the Spad 7 wing red star in the EV photo .////Pos a ? yellow/cream/ off-white background Disc ?