The lore, developed through hundreds of EU novels, says it is a cosmetic choice. We have Sith (Anakin/Vader and Exar Kun) using blue and Jedi using red (Adi Gallia and Leia Organa Solo to name two). There should be ZERO restriction on lightsaber color. Pure conjecture when they saw the first pictures of Qui-Gon and Obi-Wan in the late 90s before TPM came out. That Master = Green and Apprentice = Blue never was fact.
