After completing the game, return to MYST Island and go to the Selenitic age linking book in the spaceship. Link to Selenitic and then look up to see furry-dice hanging above you. If you then look to the floor of the ship (opposite the door) you will see a 'Cosmic Osmo' CD. Click on it to play the rest of the game in grey scale. To return to normal colours, just click the CDs again.
Link to the Mechanical Age and enter the more lavishly decorated chamber. Open the chequered box next to the throne and click the Myst CD-ROM that is inside to see a new graphics style. Repeat the process to return to normal.
After raising the cogs to get to the clock tower on Myst Island, open the door and go around the back of the tower. Click the two floppy disks at the base to see a new graphics style. Repeat the process to return to normal.