The Colour of Magic is written by Terry PratchettTerry Pratchett wrote Witches Abroad
[Human]: Can you tell me about the Colour of Magic? [Assistant]: Sure! Terry Pratchett wrote it. It falls under the genre of Speculative Fiction. Any interest? [Human]: Yes. That seems interesting. Pratchett wrote Witches Abroad which I just finished reading. Do you know when Colour of Magic was released? 
Actually, the Colour of Magic was released before Witches Abroad. It came out in 1995, whereas Witches Abroad was published in 1991.