Octarine is the colour of magic in Discworld. It has been described as "a sort of fluorescent yellow-greenish purple."
Only magical types and cats possess the octagon cells in the retina that are required to see octarine. This is evidently not restricted to the Disc, as the feline students at OFUR were made to write lines in urple ink during detentions instead of the octarine the other students had, the reasoning behind these unusual inks being that it's hard to write when the ink is either invisible or painful to look at. Furthermore, when Agents Orual and Gus accidentally turned a Potterverse Sue's potion octarine, the canons noticed and had to be neuralyzed.