Pervects perceive the color spectrum differently than humans.
Aahz describes Gleep as "blue" in the short story "Myth-Ter Right", and calls Imps "snappy dressers". Usually Gleep is said to be green by other viewpoint-characters, and Imps' dress sense is disparaged as Impossibly Tacky Clothes by others in the later books. If Pervects see the colors of things differently than Klahds and other human-types, same as he hears the name "Isstvan" differently from how Skeeve does, this would explain the discrepancy.
