As far as I have seen, the Jedi have different color lightsabers viz. blue, green...but, all the Sith have red-colored lightsabers. I don't have a heavy grip on Legends, so I can't declare it an universal rule, but look at this:
In the last episode of Season 4 of Star Wars: The Clone Wars TV series, Asajj Ventress gave her one of lightsabers to Obi-Wan Kenobi and said, "I want that back."
Then, Obi-Wan Kenobi replied, "Alright. That's not my color."
It means that Jedi don't use red-colored lightsabers. My question: Why? I don't think color has something to do with Force use or abuse. Anyone can pick their favorite colored crystal to create their lightsaber. What's the problem with red?
Anakin was using a blue-colored lightsaber that he lost in the battle with Obi-Wan. But, after becoming Vader, he started to use red-colored lightsaber. Why do the Sith use red-colored lightsabers?