Octarine

[…] colors like octarine, the eighth color, an elusive spectral mix that's hard to describe and impossible to perceive. That's the thing about color. Try describing a rainbow or a sunset to a blind person or ask a synesthete[…]

Back in ScienceFictionaLand, octarine is the color of magic, an animating force in the Discworld series by Terry Pratchett. A fluorescent, greenish-yellow purple, octarine can only be detected by wizards and cats.

Another way of looking at the magical character customization option [in which players choose their color] is that it allows players to find their own octarine, the color of their own magical self in Peter Carroll's vision of things. Octarine is a fictitious eighth color of magic invented by Terry Prachett in his novel The Colour of Magic and later used by occultist Peter Carroll in Liber Kaos to describe the color of a magician's rebellious, mischievous sorcerous self[…]

Octarine, the color of magic, glows from your finished pages.

The eight types of magic can be attributed to the seven classical planets, plus Uranus for octarine. […] Following Prachett's hypothesis (and many a useful thing is said in jest), the eighth color of the spectrum, which is the magician's personal perception of the color of magic, may be called octarine. […]