Yes, human blood is green in the deep ocean. We have to be careful about what we mean by color. Objects don't really have an intrinsic color. Rather, the color of an object is determined by three factors: 1) the color content of the incident light that is illuminating the object; 2) the way the object reflects, absorbs, and transmits the incident colors of light; and 3) the way in which the detector such as your eye or a camera detects and interprets the colors of light coming from the object. In everyday life, the incident light (such as from the sun or from a light bulb) typically contains all colors of visible light in nearly equal proportions. Furthermore, the healthy human eye can detect all colors of visible light. For these two reasons, in typical circumstances, we can treat the color of an object as only depending on the properties of the object itself. However, once we move away from typical circumstances, we have to use the more complete description of color, which involves the light source, the object, and the detector. With this in mind, let's turn to the color of blood