As this is an ideas thread it is possible to have edits covering the whole spectrum of sabre colour rationales (including what looks nice).
Though personally the blue for Jedi up until Luke in ROTJ makes a whole lot of sense to me.
When Luke loses his father's sabre it's like the end of one era and when he builds his new one it feels like the beginning of a new one and warrants Vader's fascination when he first inspects it.