We’re used to seeing the world around us illuminated by light in a full range of colors—and our eyes have evolved accordingly. A typical human eye has three types of cones, or color receptors, each sensitive to mostly reddish, greenish, or bluish light. When all three are stimulated simultaneously (as they are by the three bulbs used here), your visual system perceives “white,” rather than individual colors of light.
But colors seem to play tricks when you shift the light balance: A yellow fish may look green in one light, grey in another. The perceived color of any object you see comes from reflected light. For example, a red fish appears red because it absorbs all the many colors in white light except red, which it reflects back to your eyes. However, objects can only reflect back the light that’s available: A red object can’t look red if there’s no red light to reflect. Instead, it will simply look black.
Turning off the red bulb roughly approximates the effect of diving deep underwater. White light tends to lose its red component as it passes through water, leaving behind mostly blue and green light to illuminate the underwater environment. In this blue-green light, a red fish may look black, and a yellow fish may look green.
The altered availability of light in deep water has important consequences for aquatic organisms. Since very little red light is available, many aquatic animals have evolved to be completely incapable of detecting it, favoring detection of blue light instead.
Meanwhile, many prey organisms living at depths where blue light penetrates, but red does not, have evolved to be red. To their predators they appear nearly black, effectively disappearing in the dim light of the deep.