[Stepheninky]

if you Google tomato diseases green shoulder, you will get quite a few results.
Goodwin hit it right on the money, and its caused by lack of leaf cover.

Here is info on it:
Yellow/green shoulder

area affected is exposed to sun, and chlorophyll fails to change as fruit matures.

SteveS we posted at the same time but edited this in: Shade should help it.