Pigmentation dispersal

Pigmentation dispersal could cause the eye to change colour. When a Rutan mothership exploded over Fang Rock, Leela turned, against the Fourth Doctor's instructions, and saw the blast. The flash prompted pigmentation dispersal which changed the colour of her eyes from brown to blue and left her temporarily blind. (TV: Horror of Fang Rock)

Behind the scenes
Louise Jameson found the brown contact lenses she wore as Leela extremely uncomfortable and seriously considered leaving the show as a result. Therefore, for Season 15 the production team created an in-universe explanation that allowed her to play Leela with her natural eye colour.