ill assume everyone here has read the curse of the blue lightsaber thread, if not do so then return here.
so, luke is training on the Falcon right, and he's so strong in the force that his saber goes from blue to green cause he broke the curse..but then alderaan blows up and he looses all his focus, or maybe he just stopped to early (i mean, he blocked three bolts and was like, ok im done, when do we eat) so he lost his focus and wasnt able to keep the power/abilty to defeat vader, his saber went blue again.
Now, i know someones gonna be like, but he never fought vader in the first one. This is true, but vader came out to stop him from blowing up the death star. he was able to do so, but almost got blown up in the process. also, if he had been able to keep the green saber, then maybe vader would have stayed on the death star, or they could have had crazy one on one dog fight where luke blows up vader...ahh..never mind...this theory is getting weaker as i go on, let me stop talking out of my arse and try again...
ok, heres one that holds water....Lucas f'd up
-Darth Simon