Hi,
The "light blue wing underside" story originated with Alex Imrie, who published a very nice four-view of Büchner's D.VII (OAW) in his otherwise superb book "Osprey/Airwar 17, German Fighter Units June 1917-18." Alex had been led astray by a mistaken recollection by the old ace
Rudolf Stark, who had told Imrie that all OAW-built D.VIIs had light blue undersides.
Alex later admitted that this was wrong and he was mistaken; the undersides of the wings should simply be in the lighter pattern of lozenge fabric.
By the way, the upper surface (and possibly the underside) of the tailplane of this D.VII was simply the same blue as the fuselage. There is no reason to believe that it was green and white striped, as some kit manufacturers would have us believe.
Greg VanWyngarden