The eighth colour, the colour of magic itself, visible only to wizards and cats. Described as a fluorescent greenish-yellow-purple, octarine is the colour of the Disc's magical field. It is to the normal spectrum what the note that only dogs can hear is to sound. The number eight is magically significant on the Disc - the eighth son of an eighth son becomes a wizard, and speaking the name of the eighth spell (the Octavo) could end the world.