Why does Luke wear a black Jedi robe in ROTJ? Even if he didn't know black was the traditional color of the Sith, you would think he would model his Jedi attire after Obi-Wan, the one who told him about Jedi and started training him.
To show how close he is to turning to Darkness. Besides, Obi-Wan's robes are a little boring. I wouldn't copy them if I were Luke, either.
Luke Skywalker didn't know nothing of Sith tradition. He may have heard of the Sith but he probably didn't think much of it. In fact even in EU, Luke is never mentioned wearing anything remotely close to that of the Jedi of old. But if GL said anything about Luke's attire being symbolic of evil, I would like to read the quote.
Here's a quote about Luke's black outfit in ROTJ, but it's not addressing the same issue as was asked for quote about, but, nonetheless, it's about Luke's black outfit. This quote comes from Star Wars: The Annotated Screenplays page 248. Nilo Rodis-Jamero: "I remember George telling me that in samurai movies costumes say a lot about the characters; the way the costume is folded, the way it's tucked in is very important. So I thought, Luke has become a Jedi; he is more distant, more serious. I thought, What do gunslingers wear when they mean business? They wear black. If you look at Luke's costume, it's all derivative of Japanese designs... Ben also wore some kind of Japanese kimono. So I tailored the costume in a way that would allow Luke to fight in the action sequences."
I think he went black because it makes him look skinnier. You know all that meditation tends to put on a pound or two.
There's a quote from Mark Hamill where he talks about how he said to George that his outfit in ROTJ is very Vader, and George told him that's the point. I know I saw it in one of the Classic Moments on the OS. Here it is: Classic moment: Very Vaderish
Um, sure. He refers to his own father as a "Jedi-turned-Sith Lord" in the ROTJ novelization, and I think Obi-Wan and Yoda might've at least spent time discussing the Sith with him. The ROTJ novel and TESB seems to indicate that he knew the Emperor was quite possibly a Sith-You'd think Luke would be able to figure things out when Ben tells him it's his abilities the Emperor wants. Or when Vader refers to him as his Master.
Have anyone dismissed the fact that black is the colour of proffesionalism and coolness. he's more mature now, not that naive white tunic wearing, farm boy anymore.
I always thought it was a natural progression from ANH to ROTJ. In ANH he's this naive and innocent farm boy, which is reflected in his white clothes. In ESB, he's developing his Jedi abilities and gaining more "real world" experience, which is reflected in his grey suit. ROTJ shows him as an (almost) Jedi who is very much in control of his abilities and surroundings. This is reflected in his black clothes. Sure there are some parallels between ROTJ Luke and Vader's clothing - I thought it was common knowledge that was an intentional (as the hyperlink above shows) echoing on GL's part. Besides, as it's already been pointed out, black looks a lot cooler.
And after Luke's done fighting vader, part of his shirt is folded over, and the inside is white. Interesting. Maybe it's reversable?