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.