In J.R.R. Tolkien's The Two Towers (1954), Sméagol has a transient "green glint" in his eye when his Gollum alter ego surfaces in his consciousness.
At the word hungry a greenish light was kindled in Gollum's pale eyes, and they seemed to protrude further than ever from his thin sickly face. For a moment he relapsed into his old Gollum-manner.
Gollum was talking to himself. Sméagol was holding a debate with some other thought that used the same voice but made it squeak and hiss. A pale light and a green light alternated in his eyes as he spoke.
His natural eye color seems to be grey.
Gollum looked at them. A strange expression passed over his lean hungry face. The gleam faded from his eyes, and they went dim and grey, old and tired.
But there's always that green flash.
Gollum withdrew himself, and a green glint flickered under his heavy lids.
Changing eye color is a trope in science fiction and horror films, but when did it start? Did the trope not enter into film until Tolkien wrote The Lord of the Rings, or did it already exist in film or literature before Tolkien wrote? Who was the first to use this method to indicate that a character was not quite reliable?