In Real Life, some people have different colored irises, but what they can't have is a pupil, iris and sclera be the same color.
This trope is used to highlight the unnatural and the alien in a work. One common application is Mind Control Eyes. In some anime and manga, Blank White Eyes are used to indicate shock. In speculative fiction, Milky White Eyes are used to indicate prescience or [...] [tvtropes.org]