a) In that case you're incorrect. Not everyone would agree the practice must stop. Belonging to a religion is VOLUNTARY. So if a coloured member of the congregation is willing to follow the religion's rules, so be it. Who are you to tell him that he can't?

b) We live in the West. And the benefits of doing so are so profound that inevitably younger generations of immigrants adopt our ways. However, an additional benefit of living in the West is that no one is forced to conform to anything other than the law. If a religious group wishes to maintain their traditional way of life they're free to do so. And many, such as the Amish, do just that.