Saul,
Is this the scheme you're talking about?
Here's another shot showing several birds in this scheme. I've enlarged a portion of the photo and lightened it. It doesn't really give any clue as to the color of the undersurface of the wings. But it does show that the yellow, as per specs, wraps under the leading edge slightly. In the enlargment you can see on the lower aircraft, toward the wingtip, a lighter area which indicates this. Also, close examination shows a continuity of relative tone along the entire leading edge. This is also discernable on the higher aircraft, but to a lesser degree.
As for the color of the underside of the wing, if I had to guess, I'd say that it's the same color that's on the lower fuselage. But not being satisfied with a guess, I'm gonna dig a bit further and see if I can't find sumthin' more conclusive.
Fade to Black...