You may not know this, but Lucas asked Kershner for his permission to make the changes, even though he didn't have to. Kershner gave Lucas his full blessing and even commented that he liked the changes. I think the only one he didn't like was the scream added to Luke's fall.
However, it doesn't matter. Lucas wrote the story to Empire Strikes Back and wrote the first draft of the script. It's Lucas' vision that Kershner was presenting. So Empire Strikes Back is Lucas' work and vision.

Also there is a difference. Lucas made the Star Wars movies. He is only modifying his own work. In most colorizing cases its not the original filmmaker colorizing it.