Rule of Red

The ancient Sith Rule, where a Sith can only have a red lightsaber. This rule exists because the Jedi didn't like red, but the Sith thought red looked intimidating, so then only allowed themselves to use red lightsabers, instead of the much cooler Jedi colours. This lead many Sith to be very jealous of the Jedi, resulting in more Sith killing Jedi.