Kapheen said:

This might be blasphemously radical thinking, but I wonder if the colour of someone's lightsaber couldn't be a representation of their 'Force aura' or something like that. So as Anakin starts to slip towards the dark side, his lightsaber starts to get altered along with him until he is fully evil, and it is fully red.

There can be hints of red appearing in his saber when he kills the Tusken Raiders that abducted his mother, and again when he slices off Count Dooku's head. Finally when he enters the Jedi Temple to murder the kids his saber is full-on red.

When Obi-Wan takes back the lightsaber it technically becomes 'his', and therefore changes back to the blue colour.

This edit is theoretically in line with the cannon of the OT. And if this were true, Luke's saber would've turned red when he was fighting Vader in ROTJ. Also, I think it's pretty silly to have 'mood ring' lightsabers, but that's just me.