Countershading
Being dark on the back and pale on the belly, which hides a fish from predators looking down or up.
Also: Thayer's law
Most open-water fish, from mackerel and tuna to sharks and sardines, have a dark blue, green or grey back and a silvery or white belly. Seen from above, the dark back blends with the deep water below. Seen from below, the pale belly blends with the bright surface. The light falling from above is also evened out, so the fish looks flatter and harder to see. The American painter Abbott Thayer described the principle around 1900. Some deep-sea fish add belly lights to the same effect.