I had a long conversation this evening with my P-61 pilot source. The aircraft had four hard points, for which all could take a 1000 lb bomb, with a total flight time of 5 hrs if the bombs were dropped at some point.
The inner two points could take a tank from anywhere of 100 to 235 gallons, which is a pretty hefty 2000 lbs. Metal, not a drop tank if avoidable.
His squadron had all black a/c and the tanks were black, as all of his photos show.(He was the squadron photographer).
If they look any color other than black, our best guess is neutral gray which would have matched the undersides of the OD aircraft, or aluminum.
As for visibility, not an issue. In his theater which was the Med and Italy, the a/c would have radar search in five directions, plus were operating most of the time under ground radar control. The main nemesis was a maneuverable single engine a/c named "Hat Check Charlie" which had rear looking radar and was a tag along to the bomber formation, and tried to set up an ambush to following nightfighters.
He remembers the tanks being used as car bodies for salt flat racers.