How to find the final blue and red pages that will complete the blue and red books.
Clue from Sirrus and/or Achenar
If you have gotten all of the red and/or blue pages from the Ages accessible from Myst Island and put them into their respective books in the library on Myst, the book's captive will tell you They say that if you do so, it will lead you to the final red page, the final blue page, and a green book, which they warn you not to touch, because it will imprison you just as they were imprisoned.
Enter the fireplace in the library and press the red button on the left to close the door. Enter the following, then press the red button:
xx....xx .xx.x.xx x.x...xx x..x..xx xx..xx.. xxxxx.x.
Once you do so, you will have the choice of taking the red page, the blue page, or ignoring the warnings and opening the green book. It is recommended to save the game prior to making your choice.