Just one change to the Luke projection sequence make Luke’s lightsaber green instead of blue throw the audience off
You’d need to change the hilt to the ROTJ one too, then. I’m actually not against the idea if it can be done, but the hilt definitely needs to change if the blade color is changed.
The legend of Luke Skywalker, as he wanted to appear, used the blue lightsaber. The green one was only known by his Jedi apprentices, Kylo, Vader, Ben, Yoda and the Emperor.
At least that’s how I interpreted it. To that specific moment, the blue lightsaber had more weight.
That doesn’t make much sense, since the green one is presumably the one he used throughout the years after ROTJ, when his legend grew.
I could see him using the blue one because it is a reminder to Ren that he’s unworthy of his grandfather’s blade, but that’s about it.