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This answer showed me something I didn't know; that Luke's lightsaber was blue in the first trailer for Return of the Jedi.

As we know, Luke used a green lightsaber in the film. That same answer mentions it was to make the blade stand out better against Tattooine's blue sky, but doesn't back it up with references.

Why was the blue blade changed to green?

Of course, Luke lost his old blue lightsaber when he was disarmed by Darth Vader in The Empire Strikes Back. But that was before Return of the Jedi started production. Yet in the trailer for Return of the Jedi, we see Luke with a blue blade, not with the green blade he would use in the final version of the film.

So possibly, the original idea was for Luke to have somehow recovered his old blue lightsaber, or perhaps his newly constructed lightsaber would've been blue like the old one. But at some point between the release of the trailer and the actual film, the colour of the blade was changed from blue to green.

Another explanation:

Originally, the color of Luke's new lightsaber was blue, the same as
the one he lost in Star Wars: Episode V - The Empire Strikes Back
(1980). Luke can even be seen wielding a blue lightsaber in early ROTJ
trailers. However, when a scene of Luke assembling and activating his
new saber was cut, George Lucas thought the audience might not
understand that Luke's ROTJ lightsaber was not the same one he lost on
Cloud City. To avoid confusion, Lucas decided to change the color from
blue to green, making it clear that Luke was using a new saber. (As
part of the Expanded Universe -Legends-, Luke's original lightsaber,
used by his father and given to him by Obi-Wan, was recovered (along
with his severed hand) from Cloud City and was later wielded by a Luke
clone. Luke later recaptured this saber and presented it to Mara Jade,
his love.)

"Source": http://www.imdb.com/title/tt0086190/trivia?item=tr1643558

In-universe explanation:

During the duel with Darth Vader in Cloud City near the end of Empire Strikes Back, Luke loses the blue lightsaber that once belonged to Vader himself (which Obi-Wan Kenobi passed on to him). This happens right before the famous "I am your father" scene.

There is actually a deleted scene in Return of the Jedi that shows Luke assembling his new, green lightsaber. The scene was restored in the Blu-Ray remaster:

Star Wars fans were treated to a glimpse of bonus material, in the form of a long-lost deleted scene from Return of the Jedi. The scene has long been talked about online -- a sequence that depicts Luke Skywalker assembling his new, green-bladed lightsaber prior to infiltrating Jabba the Hutt's palace. After completing the Jedi weapon, Luke stashes it in R2-D2's dome. What makes the Jedi deleted clip so remarkable is that it made it all the way to postproduction before it was cut, so it is a rare example of a cut scene with completed visual effects and music.

The real reason for the change might indeed be to make it stand out more against the blue sky. This is parroted a lot on the web. I have not been able to find a credible original source for this claim, however.

The blue lightsaber didn't work well filming against the blue sky of Tatooine so they changed it to green. Before this, there were only going to be red and blue lightsabers to signify sith and Jedi. This then led to the other color lightsabers we see now.