It turns out that humans don't have to travel very deep underwater before their blood appears green.
Although human blood is red, it can sometimes take on a greenish-blue hue under certain conditions, such as deep underwater. The reason for this is the same reason that deep-sea fish in the ocean's twilight zone often appear red.
The explanation for this phenomenon is relatively straightforward. As you may recall from grade school science, any object we see is doing so because light reflects off of it and into our eyes. Red objects appear red because they absorb other spectrums of light and reflect red light.
That's the way it's taught when you're a kid. But it's a little more complicated than that in reality, as objects reflect a range of light wavelengths. Blood absorbs most colors and reflects light primarily in the red spectrum. However, it also reflects smaller amounts of green and blue light.
Blood in Deep Water
Meanwhile, water appears blue because it absorbs more light in the red wavelengths. When you travel deep enough underwater, red light gets absorbed enough that only blue light reflects back to your eyes, making it appear blue. Therefore, when human bloodâ€"which is redâ€"descends far enough into the ocean, it will look greenish-blue.
Because red light is absorbed so well by water, many species of fish that live at those depths are red in color. These animals are essentially invisible at those depths.
Meanwhile, black animals absorb all available colors of light, and animals that are red also appear black because there is no red light to reflect, and their bodies absorb all other available wavelengths of light. Thus, in the deep ocean, red and black animals are the most common.