What 1940 British fantasy film in Technicolor features a thief who aids a prince?

The 1940 British Technicolor fantasy film featuring a thief who aids a prince is The Thief of Bagdad.

The film follows Ahmad, a deposed prince of Baghdad, and Abu, a young thief played by Sabu. In this version, the thief and prince are separate characters—a significant change from the 1924 silent film, in which Douglas Fairbanks played the title thief. Abu helps Ahmad escape danger and ultimately becomes central to the effort to overthrow the sorcerous usurper Jaffar.

Produced by Alexander Korda, the film was directed by Michael Powell, Ludwig Berger and Tim Whelan, with major visual contributions from William Cameron Menzies. Its vivid sets, magical carpet, genie and fantasy landscapes made it a landmark of British adventure cinema.

The movie is also important technically. Larry Butler’s special-effects work used an early manual bluescreen process, and the film won Academy Awards for cinematography, art direction and special effects. Its title can be confused with the 1924 version or the 1940s-era stage and literary Arabian Nights tradition, but the Technicolor British film is the intended answer here.