Prior to the Battle of Ruusan, Jedi used lightsabers in a vast array of colors. Afterwards, Jedi relied on more common Ilum crystals such as blue and green. During his Knight Trials, Mace Windu braved the rock-encrusted world of Hurikane on the orders of his Jedi Master to obtain his lightsaber crystal. He encountered the planet's natives and, in a moment of panic, accidentally threw them over a cliff. Realizing his mistake, the young Padawan saved the natives. It was from this mission that Mace Windu was given the rare crystals as a gift. In the New Jedi Order era, Jedi had a wide array of colors such as orange, yellow, green, blue, purple, and light blue.
The synthetic red crystals favored by the Sith can create a somewhat more powerful blade than blue or green crystals but are generally not as pure because of their artificial nature and can thus be undesirable as the focus of a lightsaber. The Sith apparently had a large supply of red crystals and also favored red as a symbol for passion. It was not uncommon for the Sith masters to supply their learners with the synthesized red crystals. However, Darth Vader, at the beginning of his Sith period, wielded a blue lightsaber, as shown in Revenge of the Sith. Likewise, Exar Kun used a blue double-bladed lightsaber in the Sith War.