To avoid being recaptured by Pinpoint Division after she was helped to escape Colonel Matthews' custody by the Third Doctor, Liz Shaw, a blonde, dyed her hair black with hair dye. (AUDIO: Intelligence for War)
Lucie Miller was also a blonde. After her first departure from the TARDIS, she dyed her hair brown, (AUDIO: Orbis, The Beast of Orlok, Wirrn Dawn, The Scapegoat, The Cannibalists, Worldwide Web, Death in Blackpool) not returning to her natural colour until some time after her second departure. (AUDIO: The Resurrection of Mars)