Rimbow

The Rimbow is the rainbow created by the Rimfall, the massive, world-encircling waterfall that surrounds the Discworld, as it plunges over the edge of the world and into space, spectacularly catching the sunlight in the process. The Rimbow is one of the Disc's more impressive landmarks (which is certainly saying something), and features in Discword novels that have characters come close to the Rim - notably The Colour of Magic and The Last Hero. It is possible to see the eighth color in it.