Luke was changing the lightsaber's batteries one day and the yellow/orange colored crystal fell out and shattered all over the floor. The only spare crystal that luke had was a blue one so he stuck it in and figured the meathead Kyle wouldn't notice
As far as I'm concerned it doesnt matter to me. Maybe there is a way to chage the saber color with some sort of console command.. then you can have your orange/yellow lightsaber back
I belive (according to some better star wars fans than I am) the color of the saber's beam depends on which kind of crystal is placed inside the saber its self.
When luke lost his fathers old lightsaber in the fight with his father/vader at the cloud city, afterwards he supposidly went back to tatoone
and found a green crystal in Obi wan's abandoned hut. Thus luke could build another lightsaber which had a green beam.