The exterior color stops at end/edge of the exterior metal "lip" over the interior metal.

As Kevin posted, the fuel tank might have interior color one it, or, it might just have a (grayish ?) primer color on it. It has been suggested that the fuel tank might/might-not have been installed before the cab interior was primed/painted. The "giveaway" is the color of the interior cab metal behind the fuel tank.

Tim