I just picked up the Cyberhobby Opel Blitz, hope to start it soon. As for the belly staying grey, not to say Dario is wrong, but you never know maybe this vehicle had the paint job all over, as perhaps some did in Afrika. I say, as you are the driver of this particular truck, you can camouflage it as you feel fit.
But he probably is correct that the underside would not have been painted.