The light our brains can see — which represents only about 0.0035% of the electromagnetic spectrum. This is what we perceive as the colors of the rainbow. So what about purple? When short-wavelength (blue) and long-wavelength (red) cones are stimulated, your brain "makes something that's actually not out there in the world," Johnson said. Red and blue are on opposite ends of the visible spectrum: When the brain encounters these wavelengths, it ends up bending this linear visible spectrum into a circle. In other words, it brings red and blue together to make purple and magenta, even though that's not what light is really doing.

As a result, purple and magenta are known as "nonspectral" colors, because they don't really exist as actual electromagnetic radiation. Nonspectral colors like purple are made of two wavelengths of light. In contrast, spectral colors — red, orange, yellow, green, blue, indigo and, importantly, violet and indigo — are made of just one wavelength.