danny_boy said:

But what about Artoo in the canyon?

I was under the impression(please correct me if I am wrong) that the 1977 prints had a dusk look for that scene.

In the 82' VHS it looks like daytime.

In subsequent releases on home video (at least from the mid 90's onwards) it looks like dusk again.

Regarding R2 and the canyon, I really don't know. It's quite likely this scene was shot bright and then filtered dark somewhere along the line. If the early VHS tapes went back to a pre-filter source (and the trailer presumably also used something fairly far upstream to get unfinished lightsaber effects), then they'd have gotten the bright shot. I'm not sure there's any way to tell if the bright shot ever made it into a new batch of theatrical prints--I personally strongly doubt it, but I suppose we can only know for sure by checking those very pink prints still in existence for signs of its previous color.

EDIT: And even if all that did happen, I'd call it a production mistake or accident rather than intentional re-timing, unless I could find evidence someone actually wanted the change.