This achievement is for creating a unicorn with different colored body parts. If you don't already have a unicorn pet, go to the pet lounge (dismissing a pet if you already have four) and pick the pink unicorn named "Crystal"). Then enter the pet database and select different colors for skin, ears, tail and feet. Every color needs to be distinct, so just keep track of what you already used.
Just to be sure I also picked a different color for the pattern as well, and added the #1 set of wings (rainbow colored). But these are probably not required.
The achievement will pop after you've saved and exited the pet database.