There is a nasty rumor making its way around the interconnected series of tubes we call the Internet. The rumor was sparked by an article on The Neurostimulation Technology Portal by Liz Elliott entitled "Magenta Ain't A Colour," which has since had people exclaiming, "Fact: Magenta isn't a color," or, "Magenta is a lie." The truth is a little more complicated than that, but I assure you that magenta is not a lie—or at least not any more a lie than any other color.
See, what we call the "visible spectrum" is really a very narrow band in a much larger spectrum of electromagnetic radiation. It is visible because our eyes have cells called "cones" in the retina that are sensitive to these wavelengths—in the range of about 400–700nm—to varying degrees. Some of the cones are sensitive to longer wavelengths, some to medium wavelengths, and others to shorter wavelengths. These wavelengths correspond to (roughly) what we call red, green, and blue light, and form the basis of the RGB color model used by digital images, TVs, flat panels, and more.
As visible light enters the eye and strikes the cone cells, the cells send electrical signals along the optic nerve to the brain. This is how our body "senses" light. Our brain interprets those three separate sensations to produce the perception that we call "color."
So back to this rumor that magenta somehow isn't a color. Elliott's thesis centers on the argument that magenta appears nowhere on the spectrum of visible light, so it therefore isn't a "real" color. If you look at a standard CIE chromaticity diagram, which maps wavelengths of light according to human perception, you'll note that every point along the curve corresponds to a single wavelength of light. Magenta, as it were, lies along what's commonly called the "pink-purple line" that runs across the bottom. All colors along this line do not exist as single wavelengths. But, all points inside the "color bag" above that line do not exist as single wavelengths, either.
The truth is, no color actually exists outside of our brain's perception of it. Everything we call a color—and there are a lot more than what comes in your box of Crayolas—only exists in our heads. We define color in terms of how our brains process the stimuli produced by a mix of wavelengths in the range of 400–700nm hitting specialized cells in our eyes—"one, or any mixture, of the constituents into which light can be separated in a spectrum or rainbow," says the OED. Elliot's article might be better titled, "Magenta is not a single wavelength of electromagnetic radiation in the 'visible' spectrum, but our brain perceives it anyway."
So despite what you read on Twitter, Virginia, there is a magenta. And it's all in our heads.
- Cambridge in Colour: Tutorials: Color Perception
- Science Daily: Color Perception Is Not In The Eye Of The Beholder: It's In The Brain