Might try the search features - plenty of prior threads about how cars were painted. Be sure to be year and plant specific
what is the spec for the trunk color ?
trunk is body color ?
Same as the exterior of the car in areas painted. In the hidden, back corners you may have found bare metal of a light misting of red oxide primer
trunk is body color ?
under trunk door body color ?
trunk door hinge body color ?
Trunk lid/door was in place when the car was painted so hinges, lid and all the mounting bolts along with the trunk lid spring and mounting hardware are body color. You didn't include other details as to when and where the car was built but in general there would be to round bare areas at the dolly mounts on each side of the trunk and the bumper guard bracket rear pointing bolts would be in place and painted. Nothing else would have been installed in that area and paint was applied after the seam sealer and sound deadener applied. Type, style and product would depend again on plant
gas tank no paint on it ?
Was not installed at the time
what is the correct carpet for the trunk ?
No carpet. Mat will depend on when and where the car was built, depending on those answers could be a couple fo different ones but your not going to be able (with any ease) to find some versions . As shown above do a search and start reading