Its probably due to the fact that some darksiders infuse the synthetic crystals with dark energys from the force that i got the idea that red saber crystals are somehow tainted with the dark side of the force. But it seems that this is only the case if you do use the dark side to help creat the synthetic crystal.
In the same regards, Luke used a Synthetic crystal that turned out green for his second lightsaber (first created lightsaber) after he lost his first one and hand on Bespin against Darth Vader.
In other words, RotJ Luke's lightsaber is a green synth-crystal.