Master Sifo-Dyas said:
When I first saw the movie as a kid, and saw Luke's black outfit ... force choking the gammorean guards and how arrogantly he threatened to kill Jabba, I was afraid he had already turned to the dark side.

When I saw the early publicity photos of Luke in his black outfit - I was afraid of that, too. I think it was intentional on their part to leave it ambiguous. Then there were the teaser trailers that showed Luke standing next to Vader in the elevator, all in black- it did really look like Luke had turned.