Quote fromExtra: This is an idea I recently had, but what if every undersea mob had bioluminescence, since underwater areas are darker at night than they are on land. The area of bioluminescense could be mainly it's face, but the gloves, helmet, and boots could glow too.
Fullbright textures is what you're looking for. Infact, Minecraft already uses them. Ever notice how spider's eyes glow red in the dark? That's cause the eyes are a seperate texture that is in fullbright mode.