“For more than 20 years, the artist Christo has worked tirelessly and spent $15 million of his own money to create a vast public artwork in Colorado that would draw thousands of tourists and rival the ambition of “The Gates,” the saffron transformation of Central Park that made him and Jeanne-Claude, his collaborator and wife, two of the most talked-about artists of their generation. But Christo said this week that he had decided to walk away from the Colorado project — a silvery canopy suspended temporarily over 42 miles of the Arkansas River.”