A worker can establish a colony on any square that contains a tradable resource. If the colony is then connected to a city, that city gains access to the resource. A colony is necessary only if the resource is not inside your borders. Note that when a city's borders expand to include a colony, the colony disappears because it is no longer needed. The worker is consumed by building a colony. A worker is ordered to build a colony by pressing B or the "Build Colony" button.