In the original Star Wars trilogy, George Lucas’s intention was that Jedi would have blue lightsabers and Sith would have red lightsabers. As simple as that. End of story.
However, in Return of the Jedi, Luke’s lightsaber is green (and totally the coolest thing ever, in my personal opinion). So why did Lucas change his mind?
The reason is really practical and has to do with the final duel between Luke and Vader. The set was completely black, Vader’s costume was completely black, and Luke’s costume was completely black. The only colors on the set were blue highlights. The designers needed a lightsaber color that would really pop and contrast with all the black and blue. They decided green was the best, most vivid option.
So, as disappointing as it may seem, when the original trilogy was made lightsaber colors meant nothing. They were designed with practicality in mind.