The Answer is Octarine.

The question is, "What is The Colour of Magic that only wizards and cats can see?" Octarine is unique to the Discworld, a circular world supported four elephants, sitting on the back of Great A'Tuin, the star turtle, who swims its way through space. Terry Prachett has written a number of stories based on the Discworld. This story is primarily about three travellers, Rincewind the Wizard, Twoflower the tourist and Hrun the Barbarian, along with a magical travelling trunk. They run away for a variety of dangers including Bel-Shamharoth a H. P. Lovecraft inspired evil entity. The book has parodies of a variety of classic fantasy characters/themes and is a real hoot to read.