Colour? What colour?

Q: Can human blood look in any other colour? 

Krishna: Go down and down in the ocean. You will be surprised to see your blood in green colour (1)!

If you go very deep, it appears black!

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.

Blood mostly reflects red light. Interestingly, though, blood also reflects a little bit of green light. If we shine white light (which contains all colors) onto the blood, blood looks red since it reflects so much more red light than green light. However, if we use a light source that contains all of the visible colors except red and shine it onto the blood, the blood will be green. With no red light present in the first place, the blood can't reflect any red light. The only thing left that it can reflect is the green light. The blood is therefore green. Note that this is not a trick of the eyes. The blood is literally green. In fact, human blood is always a little bit green. We usually don't notice the green color of blood because there is typically so much more red light being reflected by the blood. But if you shine a light on the blood that contains green light but no red light, the green color of blood becomes obvious.

This is exactly what happens deep in the ocean. Water is naturally very slightly blue coloured because it absorbs some of the red light passing through. The deeper you go in the ocean, the less red light there is in the sunlight that reaches you.

Sunlight contains all of the colors of our visible spectrum; these colors combined together appear white. Red light has the longest wavelength and, therefore, the least amount of energy in the visible spectrum. Wavelength decreases and energy increases as you move from red to violet light across the spectrum in the following order: red, orange, yellow, green, blue, and violet.

As light wavelength decreases from red to blue light, so does the ability of light to penetrate water. Blue light penetrates best, green light is second, yellow light is third, followed by orange light and red light. Red light is quickly filtered from water as depth increases and red light effectively never reaches the deep ocean (2).

Without red color in the sunlight, only green light reflects from the blood. This fact can be startling to divers who get a cut while diving. Again, the blood does not change when in the deep ocean. Rather, the green color of blood that is always there becomes obvious once the brighter red color is no longer present. Since the green reflectance peak of blood is always there, blood can be obviously green anytime you have a light source with no red colour, and not just in the deep ocean.

At 30 feet or so, there is no red light in the water, so red light can’t bounce off blood into our eyes. Instead, green is reflected by green pigments in the blood that are usually hidden by the reflected red light.

If you went even deeper, the blood would change from green to black!

Red light does not reach ocean depths, so deep-sea animals that are red actually appear black and thus are less visible to predators and prey.

Colour is due to the reflection of different wavelengths of visible light. When white light (containing all colours on the spectrum) strikes an object, some wavelengths are absorbed; wavelengths that are not absorbed reflect back to our eyes. That is what we perceive as the colour of that object and it has an impact on the colouration patterns of animals in the ocean. When struck by white light, a red fish at the surface reflects red light and absorbs all other colours and thus appears red. However, the deeper you and the fish go, the less red the fish will appear, because there is less and less red light to reflect off of the fish. At 100 meters, red light does not penetrate and, at this depth, a red fish is difficult, if not impossible to see. Instead, the fish appears blackish because there is no red light to reflect at that depth, and the fish absorbs all other wavelengths of colour.

In the twilight zone, there are numerous animals that are black or red. At depth, these animals are not visible. The black animals absorb all colours of light available and the red animals appear black as well since 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 predominate.

Since the colour blue penetrates best in water, there simply are not that many blue animals in the midwater regions of the ocean – their entire bodies would reflect the blue light and they would be highly visible to predators (2).

Footnotes:

1. https://www.wtamu.edu/~cbaird/sq/2015/07/16/is-human-blood-ever-any....

2. https://oceanexplorer.noaa.gov/facts/red-color.html