After you complete the game, link to the Mechanical Age and enter the lavishly decorated chamber find a chequered box next to the throne. Click on it and find inside an original MYST CD-ROM. Click it to hear the 'link' sound effect. You can then play the remainder of the game in glorious 8 bit colours. Click the CD again to restore the original color scheme.
After you complete the game, raise the cogs to get to the clock-tower on MYST Island and open the door. Once done, go around the back of the structure to find two floppy discs at the base. Click on these to play the remainder of the game in dithered black and white graphics, similar to those in another Cyan game 'Cosmic-Osmo'. Click the discs again to restore the normal color scheme.
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.