FYI, the underhood color on my 1954 3604 is black, looks to be a satin black and the hood part numbers are (were) still visible on each side. Now I suppose the hood may have been replaced at some point but it did have what appeared to be the same multiple colors that I sanded through to get to bare metal as the rest of the cab.
I have a 54 3104 that has been in the family for many years and after careful study when tearing down, I found the following...Apparently, the factory used a black sealer coat prior to paint. Under the door hinge bolt access plates, was the black color. Rubbing through the dash trim behind windshield showed the same. Under the hood (which has factory grease pencil writing showing "transport" which was original lower body color) was same black with slight overspray from painting edges of hood where rubber bumpers are. Now the interior of my truck is the deluxe two tone blue but I assume the standard option interiors had the same treatment. My assumption is as mentioned before, the black sealer coat which was actually the final coat on the underside of the hood. Prior years the same? MT has torn down and studied many trucks. I trust his input.