I've done a lot of colour grading and i'm pretty certain boosting the contrast and/or gamma was not what turned that blue lightsaber white, especially not that very strong blue of Luke's lightsaber during the training scene visible in those technicolor frames available on ebay.

No, I think it was changed to white from the theatrical release where it was a very strong blue for a very concrete reason by GL and for all the reasons I stated above I agree with him and consider it an improvement. The greenish colour in some of the shots is clearly an SE cockup but one that is easily fixed, like I have done in the Star Wars Semi-Specialised Edition V2.2, where the lightsaber stays white throughout that training scene on the Millenium Falcon.