New roles appear; new opportunities beckon. Just ask Sean Astin, who played Samwise Gamgee in the “Lord of the Rings” movies. Astin currently stars in “The Color of Magic,” which has its U.S. debut Sunday. He had already met its witty author, Terry Pratchett, four years ago. That was when Astin was in one of his favorite places, New Zealand, to film the mini-series “Hercules.” Paul Telfer, who played Herc, began talking about how Pratchett was coming to Auckland for a book-signing. “Paul said, ‘You’ve got to meet this guy; he’s an amazing writer,'” Astin recalled. “We went down there and stood in line.” ‘The Color of Magic’ stars former Hobbit