Octarine, also known as the Colour of Magic or the King Colour, was the eighth colour of the Discworld spectrum. It was visible only to wizards and cats. It is generally described as a sort of fluorescent greenish yellow-purple (as discribed in the colour of magic). The only time non-wizards can see it is when they close their eyes; the bursts of color are octarine.
The presence of octarine generally signifies the presence of magical energy, such examples are the Rimbow, where a spell of significant level wizardry is taking place and Unseen University.