Octarine

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.