Who turned off the colors?

"Xander contracts the mysterious virus of the Rainbow-Barfing Unicorns. The virus gives the unicorns their barfing abilities, but it's draining Xander of any color at all! Xander's fate lies in the hands, er, hooves, of Ronk, the most dimwitted of all unicorns. This is not good. In fact, this is bad-- really, really bad"--Back cover.