The Osprey book comments on this:
"The upper wing seems to have been yellow on the upper surfaces at least, while the remainder of the wings were covered in four-colour fabric. The black patches on the surface of the top wing are questionable. They were probably not intended as markings, as their location over the main and auxilliary spars indicates that they were some sort of crude fabric repair, with span-wise tapes applied across them to strengthen the fabric."
I wonder if Gotthard heard a loud "crack" from the wing during one flight and the ground crew checked the webs on the spars? Maybe there was evidence of condensation inside the wing and they opened the fabric to check for damage?
At any rate, the repair was carried out after the wing was painted yellow, so for at least some of the time it was pure yellow and could be skinned like that, or as shown with the repair!
Cheers,
RD
_________________________
Beyond gliding distance