It’s just an illusion.
The story of how David Bowie’s eyes became his most distinguishing physical feature is an unusual one, as IFLScience recounts. It begins with Bowie and a friend getting into a fight over a girl whom they mutually admired, resulting in Bowie’s left eyeball getting scratched and leaving his pupil permanently dilated – a condition called aniscoria. “This can create the illusion of having different coloured eyes because the fixed pupil does not respond to changes in light, while the right pupil does. So Bowie’s left eye often appeared to be quite dark, due to the blackness of his dilated pupil, when compared to the blue of his right iris.”