Why do fish have a dark dorsal side and a light ventral side?

Most of the fish have a similar color and for good reason. The fact is that this is a kind of protective color that allows fish to avoid predators. The fact is that the dark dorsal part allows you to be invisible from above, since in this case the fish merges with the bottom of the aquatic environment in which it lives. And the light belly also allows you to remain unnoticed from below. The fish got this color in the process of natural selection. Fish with a different body color cannot survive for long, as they are very noticeable, which attracts the attention of predators.