Lightsabers come in a variety of colors, but the Sith are notorious for exclusively using the red variants of the weapon. Star Wars canon and Star Wars Legends both provide different explanations regarding the Sith's use of red blades and lightsaber colors in general. Let's break down the meaning behind these different weapons and why the Sith stick with their signature color.
Kyber crystals are the main power source for lightsabers in both canon and Legends. In canon, younglings mine for kyber crystals in the Crystal Cave of Ilum, which is an important part of the Jedi tradition. These crystals are usually colorless until they bond with a Jedi over the force, with most ending up blue or green. Other colors such as purple, yellow, black and white are known to exist, but are extremely rare.
These crystals are inherently attuned to the Light Side of the Force, and brutally resist attempts by Dark Side users trying to wield them. This forces the Sith to use a process called "bleeding" to bend the kyber crystals to their will. During the bleeding process, a Force wielder must fuse rage, hate, fear and pain into a kyber crystal by means of the Dark Side of the Force, resulting in a red blade.
In Legends, kyber crystals are also found on the Crystal Cave of Illum, but these caves exist on Dantooine and the Adega system as well. These kyber crystals already have colors attached to them, but both Jedi and Sith had to imbue them with the force before placing them in a lightsaber. Most Jedi use natural kyber crytsals, but the Sith use synthetic red ones that are filled with the negative energies of their creators. These red crystals boost a Dark Side wielder's Force and offensive abilities. Not all lightsabers are made of crystals however, as physical remains, shards of glass and even special micro machinery have been used. Luke Skywalker also used a synthetic crystal in Legends, as natural ones were unavailable on Tatooine.
Blue
Blue lightsabers are among the most commonly wielded weapons in the series, with Obi-Wan Kenobi, the Skywalkers and Ki-Adi-Mundi wielding the weapon. In Legends, blue crystals are typically given to Jedi guardians, who use the Force on a physical level.
Green
Green lightsabers are also common and have been wielded by the likes of Qui-Gon Jinn, Yoda and Luke Skywalker. These lightsabers are traditionally given to Jedi Consulars in Legends, who serve as scholars of the Force and seek to fight the Dark Side at its core.
Yellow
Yellow lightsabers were wielded by the Jedi Temple guards in both canon and Legends. Some canon yellow lightsaber wielders include the Sith Asajj Ventress and Rey in The Rise of Skywalker. Yellow crystals were typically given to Jedi Sentinels in Legends, who honed their skills in both combat and intellectual pursuits.
Purple
Purple lightsabers are rare in both canon and Legends, with Mace Windu and Huulik serving as its wielders. Windu's lightsaber had an electrum finish, which gave his blade a purple hue.
White
Ahsoka Tano is the only Jedi known to wield white lightsabers in canon, as her kyber crystal had its frequency and angle changed by Anakin Skywalker. White lightsabers are used by the Imperial Knights in Legends, who are Gray Jedi loyal to the Emperor of the Fel Empire.
Black
The Darksaber is the best known black bladed lightsaber in both Legends and canon. This weapon was created by Tarre Vizsla under unknown circumstances, but the blade has a crystal that serves as a conduit for Force energy.