Is there a reason why Kafka's Metamorphosis is in pink?

That's because it's listed in the Gutendex API as being under copyright (looks like the copyright is on the English translation. https://www.gutenberg.org/files/5200/5200-h/5200-h.htm).

Aww, my personal fave ("In Praise Of Folly" by Desiderius Erasmus) didn't make the top 20 :(