Sean Astin (Twoflower) and Sir David Jason (Rincewind) are long-term Terry Pratchett fans. Jason named "The Colour of Magic" as his choice of favorite book of all time in the BBC's Big Read survey.
The colors of the pattern on Twoflower's shirt aren't as garish as the description in the book, but they had to be chosen to avoid primary colors, or they'd have vanished in the bluescreen and greenscreen sequences.
Richard da Costa (Luggage) is also the Librarian as an orangutan. He studied real orangutans in a zoo to learn how they moved.
Sir David Jason (Rincewind) is thirty-one years older than Sean Astin (Twoflower). In the source material, on the other hand, Rincewind is implied to be younger than Twoflower. The former was in his early thirties while the latter already had two adult daughters.