What was the first Doctor Who story broadcast in color?

The first Doctor Who story broadcast in color was Spearhead from Space.

The four-part serial was broadcast on BBC1 from 3 to 24 January 1970 and opened the programme's seventh season. It introduced Jon Pertwee as the Third Doctor after his regeneration from William Hartnell's First Doctor-era successor, Patrick Troughton's Second Doctor. The story also began the Doctor's enforced exile on Earth and his regular association with UNIT.

Spearhead from Space follows the Doctor, Liz Shaw, and Brigadier Lethbridge-Stewart as they investigate the Nestenes and their plastic servants, the Autons. It therefore introduced the Autons as well as marking the programme's move into color production. The story's striking shop-window mannequins remain one of Doctor Who's most memorable early uses of horror.

A production strike at the BBC forced the serial to be recorded almost entirely on location and on 16mm film. That unusual method made it the only classic Doctor Who serial filmed wholly on film, and it later enabled an unusually detailed high-definition restoration. Planet of the Daleks was also broadcast in color, but it came later in the same season.