>>> It is a leading cause of different colored eyes
I'd have to query that statement. My mother had different coloured eyes, as did many children I encountered during my teaching career. Not a single one of them displayed any other symptoms of Waardenburg Syndrome (such as poor hearing) so, while it might be true to say that there's a higher prevalence of different coloured eyes among people with Waardenburg Syndrome than there is within the general population, it doesn't follow that Waardenburg Syndrome itself is a 'leading cause' of what, to me, is a rather attractive abnormality.
The incidence of Waardenburg Syndrome is estimated as about 1 in every 10,000 of the population. (Source:
http://www.rightdiagnosis.com/w/waardenburg_syndrome/stats.htm ). However there seems to be no reliable data available about the incidence of heterochromia within the relevant statistical subset.