Return to the LIBRARY (i.e., first door on the balcony above the MAIN HALL). Use the Interact key to press each of the protruding colored books in the right sequence: cerulean, topaz, viridian and crimson (i.e., blue, yellow, green, red).
NOTE: If you press any of the books out of order, the puzzle won't work. To reset it, just continue until you've pressed all four books. They will then pop out of the shelves again so you can start over.