In some movie about making epII Jackson asked Lucas about his lightsaber colour. Lucas said that good guys have blue and green and the bad guys have red sbars, and then Samuel L. Jackson asked Lucas could he has a purple lightsaber and Lucas agreeded.
And blue and green crystals found in lightsabers aren't only natural. In Shadows of the Empire Luke's creats his lightsaber in some sort of fireplace or somathing like that. And in my opinion red crystals are found in places strong in dark side of the Force. That will explain why the Sith use them