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[…]

2010 January 14, Peter Morville, Jeffery Callender, Search Patterns: Design for Discovery, "O'Reilly Media, Inc.", →ISBN, page 13

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.

2013 September 17, Jude Stewart, ROY G. BIV: An Exceedingly Surprising Book About Color, Bloomsbury Publishing USA, →ISBN, page 129

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[…]

2014 April 22, Jeff Howard, Game Magic: A Designer's Guide to Magic Systems in Theory and Practice, CRC Press, →ISBN, page 177

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. […]

2023 July 3, Peter J. Carroll, Liber Kaos: Chaos Magic for the Pandaemonaeon (Revised and Expanded Edition), Weiser Books, →ISBN, page 87