I don't doubt your explanation. Most likely, Lucas made the decision due to the contrasts in color. But that does make me question the AT-ATs' laser color, because that trailer proves that he himself intended to give the AT-ATs green lasers. This means that the correct and appropriate color was green, but was changed only as a cosmetic decision, with the thinking that red would stand out better on Hoth. There will certainly be people on both ends of the spectrum here, as far as which looks better, but the bottom line is that it was supposed to be green, and was changed later in production due to color contrasts with the snow/sky. Based on that, I feel that the lasers could certainly be changed to green without upsetting any continuity. I understand that Adywan is against this, so I won't be mentioning this again, but my feeling is that if Lucas had planned for green lasers, and only changed them because of how they looked against the sky, then this whole suggestion of changing them back to green was certainly not out of the realm of possibility. Myself, I thought they really jumped out in the trailer, but then I didn't really have a problem with Luke's blue lightsaber on tatooine, either. I think Lucas was more concerned about those contrasts than he needed to be.