Around 4,000 BBY, lightsaber crystals were used to indicate a Jedi's chosen class. Blue indicated a Jedi Guardian, a Jedi who used the Force on a more physical level. Green indicated a Jedi Consular, a Jedi who preferred to ponder the mysteries of the Force and fight the dark side at its heart. Yellow indicated a Jedi Sentinel, a Jedi who honed their skills in a balance of combat and scholarly pursuits.
This distinction fell out of practice in later years when most Jedi used blue or green crystals, based on the supply on the various source worlds. Purple, yellow, and orange crystals were still used by some Jedi until the the fall of the Jedi Order, but they were exceedingly rare, and often passed down through generations. The Galactic Empire banned trade and possession of all lightsaber crystals. [footnote]http://starwars.wikia.com/wiki/Lightsaber_crystal[/footnote]
