If you cut yourself (or spear a fish) underwater, the cut bleeds a bright emerald green. But as you draw closer to the surface, the blood turns brown, then pink, and finally red at the surface. Why does it change colour?

In fact, the blood doesn’t change colour at all. What changes is the light in the water.

White light, like the light from our sun, contains all the colours in the spectrum: red, orange, yellow, green, blue, indigo, and violet. A white piece of paper looks white because all the colours are bouncing off it and reflecting into our eyes. A red thing looks red because red light bounces off it, and all the other colours are absorbed. A black object is black because all the colours are absorbed and no light reflects into our eyes.

Different colours of light also travel at different speeds, and water slows light down. So as you dive down into the water, the first colour to be filtered out by the water is the slowest: red.

That means that 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.

That’s why there are so many red plants, corals, and fish in the ocean. Red is a very easy colour for the body to produce, but in white light, it stands out and is easy to spot by predators. However, 20 feet down into the ocean, there is no more red light in the water and the red fish looks black!