In A New Hope and The Empire Strikes Back, lightsabers were presented as either blue or red. Obi-Wan Kenobi's lightsaber had a blue blade, which became associated with the Jedi, as did the one that he gave to Luke Skywalker. Darth Vader, however, had a red lightsaber, which then became forever associated with the Sith. This all changed in Return of the Jedi when Luke Skywalker had built himself a new lightsaber. His new lightsaber broke away from the established color standard that everyone had come to expect as this one was green. However, all of the original posters and promotional material for Return of the Jedi had Luke Skywalker using the traditional blue-bladed lightsaber. Luke Skywalker's blue lightsaber even made it into the trailers. Based on this you can imagine the audience’s surprise when the actual film had Luke Skywalker using a green-bladed lightsaber rather than a blue one.
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