According to a German book on u-boats that I have ,it does state that "horizontal surfaces are to be painted a darker gray and vertical surfaces are to be painted a ligther shade of gray ". So yes the deck should be painted a darker gray than the hull, but not all boats did this and there are many variations to this rule. I think it was up to the ship yard and the time factor when repairing a boat ( sometimes there was very little time to spare before the boat had to leave for patrol so when repainting the boat got only one color all over above the water line etc. ) If you like weld lines put them in, many people do this but they really are not correct,only nice looking 'eye candy ". The reason here is that they would be way out of scale. A boat of this size the real welds would be invisible to see or at best a very thin line.