i have seen this before and i've got to believe it's an example of how differently lithographed surfaces (the tank sides) and painted surfaces (everything else) age, possibly even being linked to excessive UV exposure.
here is a the prewar version that shows no discoloration, so i don't believe it's strictly an age related problem...
frankly i like some of the duller finishes in the Marx 3/16" scale line.
nice find.
cheers...gary