In Bowie's case the pupils are also of noticeably and, dare I say, eerily different sizes. I don't claim there's any connection with his career, but you can imagine Dave looking in the mirror and thinking, This is not the face of a guy who was meant to drive a beer truck. Eye color is mainly a function of the pigment melanin. If your eyes have a lot of it, they're brown. If they don't, they're blue.
Green eyes result from yellowish flecks of fatty pigment against a dark background.
Some men think a green-eyed woman is exotic. The truth is she's got fat eyes. Some people's eye color does change for no apparent reason. Your brother probably saw something about the Louisville Twin Study, which rated color change on a scale of 1 to 15. Researchers concluded that changes occur in 10 to 15 percent of Caucasian adults.