The idea here is to use lamps to lighten the undersurface of the aircraft so that it offsets the shadowy effect of a darkened underneath. I would imagine that is probably pretty well classified, even if it is technology over 50 years old. The other flavor of visual masking is simply by color. An aircraft at low to medium altitudes tends to be a black dot against the background of the sky. Ever been to an air show and had trouble following the path of a fighter as it does a vertical swoop to 40,000 feet.
Chances are it was an aircraft painted a medium gray.
The gray, when combined with light scattering at low to medium altitudes ensures about as low observability as can be possible without some other very high tech trickery. As mentioned earlier, an F-4 Phantom, Compass Ghost, was painted a light blue on top and bright white on the bottom. When combined with bright lamps this aircraft was seen to reduce visible detection by 30 percent. Okay, so we've gone from fancy materials to color and lights to offset the visibilty of the underneath of the aircraft. What about from above.