Jeff,
Fabio is quite correct, usually the undercarriage, including wheel wells and inner face of doors, and wheel hubs on US Navy aircraft were painted the underside colour. Earlier, when aircraft were painted in the 3 colour scheme, the undercarriage would have been painted white. There is possibly a start date for this to have been started but I don't know when that was. I think drop tanks, in general, would be painted in the underside colour. Of course there were exceptions to this and drop tanks painted for use on one aircraft, of one colour, may sometimes have been used on a similar aircraft painted a different colour, or maybe going through a transition. The drop tank on my Hellcat is actually painted white. This may have been left over from the earlier 3 colour scheme or might have been painted as part of the overall scheme as this aircraft has other white elements on it such as white stripes on the tail. You are also correct, sometimes drop tanks would be left in bare metal.
Fabio,
If you like I can post a couple of picks of my Hellcat, it has the exhaust staining I was talking about? Or alternatively I can download them to my gallery and you can view them there. I don't want to highjack your post
Mal