I'm not completely sure, but the photos of existing DUKW's I've seen were painted a very dark, semigloss or satin OD; Not really a flat olive (I have a jar of model master that is
this close, but there's paint on the color code

), but more of a rally dark shade of blue/green. Only thing I can think of that may help you is if you can find pictures of the tank static diplays outside the Patton museum at Ft. Knox; The T-28 tank and a few others seem to have the
exact same paint color...