I believe the fuel barrels were galvanised with iron bands, and were left unpainted. The army ones certainly were.
Early war I think the kit was Panzer grey and late war it was Dunklegelb, much like army kit again. Small kit could well be RLM 02 though.
I was going with the Dunklegelb anyway as i knew that was a late war thing (although sometimes the Luftwaffe painted their equipment camo colours). I was just wondering for the primer as if i wanted to add scratches to kit then i'd need a lower surface colour.
As said probably red oxide. If sorftskins follow armour red primer and panzer grey for everthing till about 42 I think, then red primer dunkelgelb till late war.
Given how widespread red lead oxide primers were until fairly recently, I’d say it’s a safe bet that Luftwaffe ground equipment was finished in it as well.