a tough car to get a good picture, what you're seeing is mostly the reflection of the background in the lower half of the tank sides...
it's definitely not a perfect match for the silver painted ends and perhaps a factor in why the postwar versions changed to a black dome and frame only leaving the ends to lessen this distraction.
one of the more prominent Marx sets typically found with a similar overcoat discoloration was the NYC articulated streamliner. headed up by a grey painted Merc, the lithographed cars can be found anywhere between a very closely matched grey to the extreme end of the discoloration scale, closer to an olive drab tint. a very touch set to match if you're trying to put one together from individual car purchases.
cheers...gary
