Octarine is the eighth colour of the rainbow, the colour of magic. 

"It was alive and glowing and vibrant and it was the undisputed pigment of the imagination, because wherever it appeared it was a sign that mere matter was a servant of the powers of the magical mind. It was enchantment itself. But Rincewind always thought it looked a sort of greenish-purple.”

― Terry Pratchett, The Color of Magic