Colonies were city-states founded from a mother-city. Bonds between the colony and the mother-city ("Metropolis") remained close. There were occasions where the colony itself would colonise.
Greek colonies (apoikiai)
In Ancient Greece, colonies were sometimes founded by vanquished peoples, who left their homes to escape subjection at the hand of a foreign enemy; sometimes as a sequel to civil disorders, when the losers in internecine battles left to form a new city elsewhere; sometimes to get rid of surplus population, and thereby to avoid internal convulsions. But in most cases the object was to establish and facilitate relations of trade with foreign countries.
There were two similar kinds of colonies, apoikiai and emporia. The first were city-states on their own; the second were Greek trading-colonies.
The Greek city-states began establishing colonies around 800 BC. Among the earliest of the Greek trading emporia were Al Mina in northern Syria and the Greek emporium at Ischia (Pithekoussai) in the Bay of Naples, both established about 800 BC.
Two flushes of new colonists set out from Greece at the transition between the "Dark Ages" and the start of the Archaic Period, in the early 8th century BC and a second burst of the colonizing spirit in the 6th century BC. Population growth and cramped spaces at home seem an insufficient explanation for the phenomena.
Several formulae were generally adhered to on the solemn and sacred occasions when a new colony set forth. If a Greek city was sending out a colony, an oracle (before all others that of Delphi) was almost invariably consulted beforehand. Sometimes certain classes of citizens were called upon to take part in the enterprises; sometimes one son was chosen by lot from every house where there were several sons; and strangers expressing a desire to join were admitted. A person of distinction was selected to guide the emigrants and make the necessary arrangements. It was usual to honor these founders of colonies, after their death, as heroes. Some of the sacred fire was taken from the public hearth in the Prytaneum, from which the fire on the public hearth of the new city was kindled. And, just as each individual had his private shrines, so the new community maintained the worship of its chief domestic deities, the colony sending embassies and votive gifts to the mother-city's principal festivals for centuries afterwards.
The relation between colony and mother-city (literally metropolis) was viewed as one of mutual affection. Any differences that arose were made up, if possible, by peaceful means, war being deemed excusable only in cases of extreme necessity. The charter of foundation contained general provisions for the arrangement of the affairs of the colony, and also some special enactments. The constitution of the mother-city was usually adopted by the colony, but the new city remained politically independent. If the colony sent out a fresh colony on its own account, the mother-city was generally consulted, or was at least requested to furnish a leader. The cleruchs (klêrouchoi) formed a special class of Greek colonists. The trade factories set up in foreign countries (in Egypt, for instance) were somewhat different from the ordinary colonies, the members retaining the right of domicile in their own fatherland and confining themselves to their own quarter in the foreign city.