This article is useless. Magenta is a combination of colors, and is not represented by any single wavelength of light. That doesn't mean it doesn't exist. Saying magenta doesn't exist is like saying green paint doesn't exist, because it's a combination of blue and yellow (within subtractive primaries).
From wikipedia:
Magenta is a purplish red color evoked by lights with less power in yellowish-green wavelengths than in blue and red wavelengths (complements of magenta have wavelength 500–530 nm).[1] In light experiments, magenta can be produced by removing the lime-green wavelengths from white light. It is an extra-spectral color, meaning it cannot be generated by a single wavelength of light, being a mixture of red and blue wavelengths. The name magenta comes from the dye magenta, commonly called fuchsine, discovered shortly after the 1859 Battle of Magenta near Magenta, Italy.
Still, the article should have been more careful about terminology, especially in the title. Color is a description of human visual perception, of how a set of light amplitudes and frequencies is perceived. A photograph displayed on your computer monitor has different frequencies from light given off by the photograph's subject, but it has the same colors.
Magenta is definitely a color. What the author meant to say is that magenta can't be a single frequency -- but that title wouldn't have gotten as many clicks.
There's a difference. A chord is always composed of multiple notes. There's no single note that can sound like a chord. But a colour like yellow can be generated by a single wavelength or a combination of wavelengths that average out to yellow (so long as those wavelengths are not too far apart).
So in colour there is no sharp dividing line between a 'chord' and a 'note'. Some 'chords' look like 'notes' and some 'notes' look like 'chords'. Magenta is interesting because it's always a 'chord'.
It's a symmetric relationship. If a note looks like a chord then a chord looks like a note. Both a mix of frequencies and a single frequency can produce the same colour, so they both look like each other.
Basically any note played on an analog instrument has a set of overtones of various frequencies. If these tones were played a bit louder, you would hear a chord rather than a single note.
A single note played on an acoustic instrument is itself (typically) a compound of multiple frequencies, usually (but not in e.g. a church bell) in a more or less harmonic relationship.
Furthermore, the overtone series for a nontonal instrument exists; you can tell which of two blocks is higher pitched. But, the overtone series is not the regular one we expect from (half)?open pipes. So it is not treated in the brain as a tone.
Since when is it necessary for a color to be generated by a single wavelength of electromagnetic radiation?
A good way to look at colors their perceptual relationships such as complimentary color is the color wheel used in the HSL and HSV color models. Magenta is at 324° by the way; that's between red and blue (more towards red). Going to the opposite side of the color wheel (324°-180°=144°) gives you (surprise) green, the complementary color to magenta.
Magenta happens to be the area where the color wheel "joins" the opposite ends of the visible electromagnetic spectrum. There is nothing mystical about that fact which explains why greens is the only color that has a complementary color that is not made from a single wavelength.
From wikipedia: Magenta is a purplish red color evoked by lights with less power in yellowish-green wavelengths than in blue and red wavelengths (complements of magenta have wavelength 500–530 nm).[1] In light experiments, magenta can be produced by removing the lime-green wavelengths from white light. It is an extra-spectral color, meaning it cannot be generated by a single wavelength of light, being a mixture of red and blue wavelengths. The name magenta comes from the dye magenta, commonly called fuchsine, discovered shortly after the 1859 Battle of Magenta near Magenta, Italy.