Producers made a deliberate choice to color the different worlds to help the audience track the different timelines Eve and Flynn traveled: green for action-adventure, blue for science-based, and red for magic-based.
Ezekiel calls the ghosts "deadites," a reference to the Evil Dead franchise of Sam Raimi. Interestingly, Bruce Campbell, the star of the Evil Dead trilogy, appeared in the Christmas episode, And Santa's Midnight Run--as Santa!
Flynn Carson whistles as he is coming down the stairs in the Annex; the melody is the main theme of the Librarian franchise.
Jerry O'Connell, who plays the younger Lancelot, starred in the 1990s television show Sliders (1995) which features an alternate time-lines/parallel dimensions theme similar to that of the "The Librarians" season finale.