Think the the underside of panels will differ from colour to colour, should be painted in the undercoat/base coat for the cars 'finished' colour, as is the rest of inners, then they get the top coat and laquer over just the outside door wells etc. (bits you see).
Tight german....(or in the zeds case american built ) gits. Most manufacturers only 'finish' the top sides of cars now days!
If you've got a shiny painted underside of the bonnet, chances are that it's been re-sprayed :roll:
Oh btw, inside of mine (sterlin Grau) is dark matt grey.