From Thinkgeek:

To build a lightsaber, a Jedi or a Sith must first harvest a colorless crystal. It is the crucial final test to advance from apprentice to fully-fledged Force-wielder. This Lightsaber Crystal Growing Lab gives young apprentices the chance to grow their own crystals. When a Jedi or Sith attunes the colorless crystal to the Force it gains its color, and so is the case with this display, which lets you choose between blue or red light for your crystals, or the option to switch back and forth as the Force moves you.

[Lightsaber Crystal Growing Lab]