Boeing 314

The pictures I have seen also look like dull metal- aluminum I would guess.  Appearance would depend on the alloy used.  I think most flying boats did paint bottom.  But where they stop depended on plane and airline.  I have done one 314 (the Airfix kit but plan to do the Minicraft also), and added some flat grey to Testors aluminum.

 There are chemical conversion processes that still retain aluminum color, but tint or darken it.  Don't know if Boeing used that or not.

 

The Boeing 314s appeared in both natural metal and silver paint at various times in their careers, as well as in camouflage duiring the war.  There are many very good reference photos available-some of the books on the history of Pan Am have some great ones.  The majority of the time these aircraft were seen in a natural metal finish with antifouling paint on the lower hull.