Steve Nelson wrote:
The countershading was Medium Green, and was used on the leading and trailing edges of many USAAF aircraft to "break up" the outline. The Medium Green was actually lighter than the OD when applied, but the OD faded much faster, leaving the Medium Green appearing darker. The center portion of the vertical fin also often looks darker on wartime C-47s..I'm assuming because they were painted separately or by a subcontractor. The outer wings and vertical tails of OD B-17s were also often darker than the rest of the aircraft, because they were pre-painted by a subcontractor.
OD has been notoriously difficult to properly pin down, because of weathering the the field and variations in the color itself. Researcher Dana Bell once noted that the USAAF was primarily concerned with paint adhesion, rather than a precise color math, and never rejected a batch of paint because of color.
SN
It's hard to tell for sure, but I have always attributed the center section of the vertical being darker due to an over zealous paint sprayer painting the medium green on the leading and trailing edges as the vertical gets the scallops just like the wings and horizontal/elevators.