Did you ever notice in the last half of Return of the Jedi that Luke Skywalker is fully dressed in black clothing? It's a lot of black like a certain Sith Lord who has turned to the dark side.
Yes, Luke is dressed in black by George Lucas to show the viewer how close the character is to letting his fear and hate overwhelm him. He is surrounded by the dark side of the Force and is basically on a razor's edge and could turn to the dark side of the Force with the slightest provocation.
That, of course, was Emperor Shiv Palpatine's plan – to make him face Vader and try to turn him by forcing him to tap into his dark side to beat his father.
Luke suffers. He suffers from hatred for his father who cut his arm off. He suffers angst for Obi-Wan who lied to him about his true heritage. He suffers the fear that his sister Leia will be Palpatine‘s next target. He is angry. You saw him Force choke two of Jabba's Guards, right? Very angry. And, most of all, he fears that all will be lost.
But everything turns out well when Vader throws the Emperor down the shaft that's an inexplicable safety hazard in his throne room.
So, if Luke wins his fight against Vader and then turns his back on the Emperor's offer to join him, how do we really know that Luke is not joining the dark side?
Take a look at his black tunic costume. Look at the fold that became unbuttoned during the fight sequence with Darth Vader. What is that color? It's white. It's the yang to black's yin.
Underneath all Luke's stresses about saving his father and the universe, he was always the good guy – always the boy who never got to Tosche Station to get his power converters. He'd become a Jedi, like his father before him. Nice symbolism George Lucas and director Richard Marquand!
This flap of white was a very subtle point and perhaps lost on many a viewer on their first viewing of the movie. It wasn't easy to spot, but it was there to demonstrate that, on the inside, Luke was always the good guy.
In the long run, it became common knowledge to many Star Wars fans, so when George Lucas got around to writing Revenge of the Sith with Johnathan Hales, he made an even better reference to light and dark:
Recall the scene in which Obi-Wan Kenobi is about to hunt down that dastardly Grievous General robot chap. Obi-Wan and Anakin are saying their farewells. Obi says “Goodbye, old friend.”
It's the last time the two Jedi will ever talk to each other as friends. Look carefully at the lighting – Obi-Wan is in the light, Anakin in the dark. Yin and yang. A good guy and a soon-to-be bad guy. A Jedi and a Sith.