I my current game I discovered two clusters of 7 sources of spices next to a shore about 12 squares away from my borders. So I built 7 workers, built a road to the spices, and built 7 colonies. Several turns later two French boats dropped off 2 settlers and they established two cities right over my colonies effectively stealing my all 7 sources of spices without declaring war
I didn't realize this could be done. If I had built 2 cities over the spices the French would not have been able to steal them without starting a war or without surrounding my cities with other cities that have strong cultures. I don't see any advantage in building a colony over a city?
I my game it would have required less resources to build 2 cities as opposed to 7 colonies (a 4 population loss as opposed to a 7 population loss).

