Why are some dogs scared of darker people?

It probably has a lot to do with smell. People of different nationalities and races eat different foods and as a result have a different smell to their bodies. Most people don't notice the difference but we have much weaker sense of smell than a dog, although some vegetarians claim that people that eat meat don't smell very good to them. He is probably reacting just because the smell is unfamiliar. If you remain calm when he reacts, he will probably change his behavior after a while.