Actual explanation: these squid are transparent normally, but can turn on a dark pigmentation when that is a more effective camouflage. Being transparent works quite well most of the time, but if the predator has its own light source (as several deep-sea predators do) then their transparent state becomes a problem, because it’s relatively reflective compared to the water around them. In this situation, turning on the dark pigment helps them blend in with the dark water better.

Source: Zylinski and Johnsen “Mesopelagic Cephalopods Switch between Transparency and Pigmentation to Optimize Camouflage in the Deep”