The Cinematheque series “Audrey Hepburn: Sophisticated Lady” continues this week with Stanley Donen’s 1957 musical. If it’s not a great movie, it is a great-looking one. Hepburn plays a nerd who catches the eye of photographer Fred Astaire, who promptly turns her into a famous model. The sets, costumes, and songs are all splendid, as is Donen’s direction, which brings to life scenes straight out of fashion magazines. Film geeks, take note: The Cinematheque is showing a three-strip Technicolor print borrowed from the Academy of Motion Picture Arts and Sciences. So this is prime stuff.