A lightsaber’s color could be based on how often it is used, with the crystal wearing down and displaying ever redder colors over time:
I think a lightsaber colour should reflect the personality of the builder. Which is another reason why it would’ve been better for Luke to have had a red lightsaber in SW/TESB and Vader a blue one; Luke is passionate, impatient, and impulsive, while Vader is calm, cool, and collected.