Answer: Countershading

Countershading is a particularly clever form of camouflage wherein an animal (or object) is shaded in such a fashion that viewing the animal in its natural orientation leads to the animal blending into the background.

A classic and perfect example of countershading in the animal kingdom is that of the coloration found on many sharks and whales. The grey reef shark, for example, is shaded a greyish blue color on top of its body but its underside is a light white color. When viewed from above, the shark blends into the murky and dark water below and when viewed from below, it looks lighter like the sky and heavily illuminated water near the surface.

The same kind of shading has been applied to fighter jets (and other military craft) over the years such that the planes are lighter colored on the bottom (often painted blue like clear skies) and darker colored with patterning on top (to mimic the varied textures and darker colors of your typical terrain).