Answer: Gloss White

Many nuclear bombers in the United States Air Force, Navy, and other air forces around the world are painted a brilliant gloss white on the underside (and often in entirety).

The purpose of this paint scheme is not to camouflage the plane in any way (although over the years some military aircraft have had their undersides painted soft blues and pinks in order to blend in with the sky). The purpose is belied by the name of the paint, “anti-flash white”. The paint is intended to reflect some of the thermal radiation from nuclear explosions to protect the aircraft and its occupants.

This is, interestingly enough, the exact same reason the Concorde was painted gloss white: to deal with thermal radiation. In the case of the Concorde, however, one of the two main sources was much more distant and much less intense than a nuclear weapon. The planes were painted white to help keep them cool as they baked in the heat of the Sun and picked up aerodynamic heating due to supersonic flight at Mach 2.