According to an interview with Louise Jameson she was told that the name Leela meant 'dark-eyed beauty' so she was given red contact lenses which, combined with her blue eyes, made them appear brown.
"There was always an hour and a half in make-up before I was allowed on set. I wasn't black but it was more than a tan. One of my sons is mixed race – dual heritage we say now. I think I was meant to have that kind of skin."
The contact lenses lasted for three stories but limited her vision so the producer promised she could stop wearing them and, in her fourth story 'Horror of Fang Rock', writer Terrance Dicks wrote a scene to explain them turning blue. Leela's eyes suffered 'pigment dispersal' after she ignores the Doctor's warning and watches the explosion of the Rutan spaceship.