Which Kubrick film was his first in color?

Stanley Kubrick's first film in color was Spartacus.

Released in 1960, Spartacus was Kubrick's fourth feature film and followed the black-and-white productions Fear and Desire, Killer's Kiss, and Paths of Glory. It was an expansive historical epic about the Thracian gladiator who led a slave revolt against Rome during the Third Servile War.

Kubrick took over the production from Anthony Mann after about three weeks of filming. Kirk Douglas, who starred as Spartacus and produced the film, had previously worked with Kubrick on Paths of Glory and brought him onto the project. Spartacus was also Kubrick's only film as director in which he did not have complete artistic control.

The film's color photography was designed for epic scale, including large battle scenes filmed in Spain with thousands of Spanish soldiers serving as Roman troops. It earned four Academy Awards, including Best Cinematography—Color. Later Kubrick films such as 2001: A Space Odyssey, A Clockwork Orange, and Barry Lyndon were also in color, but they came afterward.