Star wars ????????????

why is it that in ANH, Luke appears to have a green saber at a part of the film? (2004 dvd).http://images.google.com/imgres?imgurl=http://www.dvdactive.com/images/editorial/screenshot/2006/9/falconsaber1977.jpg&imgrefurl=http://www.dvdactive.com/editorial/articles/star-wars-the-changes-part-one.html&h=243&w=565&sz=51&hl=en&start=3&um=1&tbnid=08GREAIay_WKPM:&tbnh=58&tbnw=134&prev=/images?q=luke+skywalker+training&um=1&hl=en&rlz=1T4DKUS_en___US208 (scroll down a bit)