Below about fifty meters, red has already disappeared. Water absorbs the long wavelengths near the surface — most of the red is gone by twenty meters, and by fifty there’s none of it left to reflect. A creature that’s red in shallow water becomes black down there.
So in the twilight zone, the band from two hundred to a thousand meters, half the animals are red. Shrimp, small fish, jellies — bright red where you can see them, perfectly black where they live. The other common strategy is transparency: glass-clear flesh, organs you can only see when light catches them wrong. Two ways to be unseen: become a color light can no longer find, or have a body that doesn’t stop light at all.
Up here, red is the color of attention. Apples, blood, traffic lights, fire. Down there, it’s the color of disappearing.