The first color episode of Doctor Who aired in 1970, meaning that the first two Doctors spent their entire runs in black and white. One fan decided to give William Hartnell's First Doctor a moment in color, colorizing 2,700 frames of the Doctor's farewell to his granddaughter, Susan.
YouTube user farfrombeingallover, who goes by MrBiscuitESQ on reddit, provided the color for the Doctor's goodbye from the 1964 serial "The Dalek Invasion of Earth." He's also involved in the recreation of the lost Doctor Who episodes. You can watch a segment from "The Celestial Toymaker," the episode he's currently working on, here.