In Star Wars- Episode IV- A New Hope Luke's lightsaber has a light blue color, almost white, however in the prequel films the same blade is a dark blue instead.
Here's the blade in A New Hope:
and here it is in Revenge of the Sith
Is there an in universe reason for this change in color or is it simply a byproduct of the newer out-of-universe production techniques of making the lightsabers?
Update: The same lightsaber is once again deep blue in The Force Awakens: