Mysteries of the Red Light-saber

Why do Sith only use red light-sabers?Find out next on.. The Mysteries of Star Wars :seoson one:light-sabers.Well,I believe it is because sith are united,just like the Jedi.But the Sith would like to show that.The Jedi however,are trying to express rank and personality.Or,Goerge Lucas wanted you to be able to tell the difference.Or the Sith use a different crystle from a different,darker,eviler,planet.Maybe even the planet where they hold their head quarters.It may be that those crystles are more powerful than a purple lightaber's crystle.Or because red represents the devil.Or because red is the color of blood.Maybe its a rank?