I just stumbled upon an article which claims "Magenta Ain’t A Colour." The author makes this claim based upon the fact that there is no specific frequency in the visible light spectrum that corresponds to magenta. She then states that magenta is basically the brain attempting to make sense of light coming from both ends of the spectrum at once by "inventing" a new color. "Magenta has no wavelength attributed to it, unlike all the other spectrum colours," she claims. " The light spectrum has a colour missing because it does not feel the need to ‘close the loop’ in the way that our brains do."
Well, by that logic, brown isn’t a color, either. Where’s brown on the spectrum? I don’t know, either. But, I think we can all agree that this site is brownish, right? Well, look closely at your monitor. You’re actually seeing a mixture of red, green, and blue light. The retina contains three types of cells, called cone cells, that are each sensitive to a different range of wavelengths. One type is most sensitive to yellowish-green, that fluorescent-looking color that has been adopted by some street signs and fire engines. Another is most sensitive to blue-green, and the third is most sensitive to purplish blue. Among them, they can sense any wavelength of light in the visible spectrum… rather like your computer monitor in reverse.
But what does this have to do with magenta? Well, find something printed in color, such as a photo in a magazine, and look closely at it. Use a magnifying glass if you have to. You’ll see not solid colors, but a pattern of dots. Each dot is most likely one of four colors: black, yellow, cyan or magenta. Each of these inks were chosen for color printing since each ink’s pigment absorbs a certain part of the spectrum (as opposed to emitting it as your computer monitor does) and when printed in different proportions, different combinations of wavelengths of light are reflected from the printed page.
So we sense magenta, and every other color, as a stimulation of the three types of cone cells in a particular proportion. In that sense, all colors, even pure yellow-green, blue-green, and purplish blue, are inventions of the brain. In this light, the idea that a color may consist of one and only one wavelength of light just seems nonsensical.