Underwater color depends on the light that reaches an object
Blood looks red in ordinary daylight because hemoglobin—the oxygen-carrying pigment in red blood cells—interacts with incoming light in a way that sends more red light back toward our eyes than many other colors. But an object cannot reflect a color that its illumination scarcely contains.
Water removes colors unevenly as light travels through it. Red wavelengths are absorbed more readily than blue-green ones, so deeper water tends to have a cooler, bluer-green light. By around 50 feet, depending on conditions, much less red light may be available to illuminate blood. Its familiar red reflection weakens, and the surrounding blue-green light can dominate what reaches a viewer, making the blood look greenish.
The same blood can look different under a new light
The color is not changing inside the body. What changes is the mix of light illuminating the blood and then reaching the eye. Shine a light with a strong red component on it, and red can become visible again—even underwater. A lamp that lacks red wavelengths cannot reveal that color as effectively.
The depth is only a rough guide: water clarity, daylight, and the kind of lamp all affect which wavelengths remain. The memorable point is that underwater color is partly a property of the light path, not just the object. Blood can appear greenish in one underwater scene and red in another, even when nothing about the blood itself has changed.
