Heterochromia iridum is a pigmentation of the irises (the coloured bit of your eye).
For some people it is very subtle, some very distinct. Some appear as totally different colours, others may have dark coloured dots in their iris. It doesn't effect the eyesight of the person in most cases at all. It can be caused by many things. Genetics, illness or disease or injury.
You can find more about it here
David Bowie is NOT one of them
But here are some celebrities that do have amazing different coloured peepers
Some actors choose to use contacts so their eyes match so it isn't always noticeable. Other times, their images are photoshopped.
Robert Downey Jnr
Alyson Hannigan (Buffy, How I Met Your Mother)
Olivia Wilde (Tron, Rush)