90% of deep-sea marine life produce bioluminescence. Humboldt squid flash patterns of colored bioluminescent lights to communicate with conspecifics using a sophisticated language.
Visual signaling is affected by both the amount of available light, and how well certain wavelengths travel. Blue light (475 nm wavelength) travels well through seawater but longer (infrared) and shorter (ultraviolet) wavelengths get filtered out. Hence, many reef fish are blue or yellow, or have striking stripes.
Watases lanternfish are hunted by predators that strike from below. Light-sensing cells around their bodies provide the essential information for ventral (downward-pointing) bioluminescent cells to make the lanternfish invisible to those looking up. Likewise, some squid employ bioluminescent bacteria for counter-illumination, to match the overhead environmental light as seen from below.
Some sea creatures, such as the black dragonfish, prevent being exposed by bioluminescence by sporting light-absorbing skin which renders them pitch black. A thin film that covers their skin comprises an intricate, layered matrix of microscopic granules that create a labyrinth for light.
The trick to being really dark is to control the scattering of light. You have to let light into a material and let it bounce around a lot. ~ American zoologist Sönke Johnsen