I agree. If I was building your model I'd be doing unpainted undersurface too.
Ullmann's analysis is pretty good. There is only one point I would quibble and that is the underside colour. He quite rightly quotes Sammelmitteilung Nr 2 of 15th August 1944 deleting RLM 65 and assumes that Dornier thereafter moved to RLM 76 but there is not much evidence for that.The 1973 inspection of the fuselage from the A-0 240102,VG+PH, now forming part of the composite aircraft on display in the US is debateable to say the least. There are some good colour photos of the fuselage,parked outside shed 22 at Silver Hill and in a pretty sorry state,taken in the '70s,which are certainly not conclusive.
Dornier's aircraft production was a commercial enterprise,not a patriotic duty, and as a manufacturer of bomber aircraft they may well have had stocks of RLM 65 to use up. D.(Luft)T.2335 A-1,the Do 335 handbook,issued after the Sammelmitteilung still lists the underside colour as RLM 65. It is at least possible that they continued to use this on various pre-production aircraft. Had the aircraft entered production,entailing BAL acceptance,then Dornier may have had to make the change to RLM 76 as per the earlier instruction.
Cheers
Steve