Answer:
Exploiting natural resources - this is the most straightforward approach. Colonies were established mainly to obtain the economic benefit from exploiting resources such as lumber, ore, gold, coal, etc.
Exploiting the population - exploiting the labor power of people in the colonies is also a way for the colonizer country to make money out of the colonized territory. The forms of exploitation can vary from straight up slavery, to serfdom, to wage labor with extremely low pay.
Making the colony a captive market - The colony can also become a captive market for the colonizing country. This means that the people in the colony are obliged to buy goods and services from the colonizing country due to internal or external restrictions to competition and trade.
