From Discworld & Terry Pratchett Wiki

Octarine, the colour of magic, the magical colour, it's...it's sort of...well, try describing mauve to the colour-blind. Most of us just have no frame of reference. It's the eighth colour of the Discworld rainbow, where we might expect ultra-violet, but Discworld light is odd stuff anyway, travelling thousands of times slower than Einstein's standard. We're not sure how Discworld eyes work at (presumably) much lower frequencies, but octarine is invisible even to most inhabitants of Discworld. The eyes of Wizards, however, contain, besides the usual rods and cones, octagons that detect octarine. They try to describe it most often as a kind of greenish-purple (doesn't help, does it?)

It is called the colour of magic because magical discharges are typically accompanied by emanations of the highest-energy light – octarine.

Less magically-sensitive humans can nevertheless see where octarine should be: 71-hour Ahmed points out the blackness around the edges of the magical fire that engulfs their ship to its captain during the events of Jingo. Here, he says, it is octarine but as we can't see it, it appears as a hole in space.

See Infra-black from Good Omens.

Let's get octarine on the periodic table!

Element 117, ununseptium (Hundredandseventeenium) was confirmed to exist in January 2016. An online petition has been circulated for it to be formally called Octarine, symbol Oc, to be pronounced ook, in honour of Terry and the Discworld. Sign here.

The petition reads: This petition is to name element 117, recently confirmed by the International Union of Applied Chemistry, as 'Octarine', with the proposed symbol Oc (pronounced 'ook'), in honour of the late Terry Pratchett and his Discworld series of books.

The Discworld series has sold more than 70 million books worldwide, in 37 different languages. Terry Pratchett died in 2015 and his final book, The Shepherd's Crown, was published in the same year. He was well-known as a lover of science and, with two well-known science writers, co-wrote a series of four books called The Science of the Discworld, which took a sideways look at 'roundworld' (Earth) science.

Octarine, in the Discworld books, is known as 'the colour of magic', which forms the title of Pratchett's first ever Discworld book. According to Disc mythology, octarine is visible only to wizards and cats, and is generally described as a sort of greenish-yellow purple colour, which seems perfect for what will probably be the final halogen in the periodic table. Octarine is also a particularly pleasing choice because, not only would it honour a world-famous and much-loved author, but it also has an 'ine' ending, consistent with the other elements in period 17.

Octarine is being counted as 'a mythological concept' under IUPAC rules, which state that elements must be named after "a mythological concept or character; a mineral, or similar substance; a place or geographical region; a property of the element; or a scientist". The Discworld stories are certainly stories about gods and heroes, and 70 million books surely count for something.