Here is final answer to the question of the camouflage scheme. I emailed Aberdeen Proving Grounds for information and I received this reply:
Sir: The original paint scheme on the gun was "Panzergrau" which, for the
lack of a better term, is black. The gun was painted in its current scheme
before it went to Italy. It is the then current Med camouflage scheme.
After firing into Anzio the Gun Captain was informed he was to be withdrawn
and as he put it "go back to the Army." The gun crew broke into a shed
alongside the tracks and found some light gray paint that belonged to the
Italian rail company. They used it to paint the gun, cutting the paint with
gasoline. After they got it painted, the German propaganda folks from
SIGNAL showed up and took several photos of the gun painted light gray and
filmed it "in action" in this totally unauthorized paint scheme. Thus, when
it was captured it was in a very light gray paint scheme and that scheme was
maintained over the years here at APG. Several years ago when we go ready
to repaint the weapon, we found the original scheme under the gray. We
marked it with chalk and reproduced it as close as we could get it.