Ancient colonialism had different forms (e.g. settlement).

Modern colonialism (Spain, Portugal, Netherlands, Belgium, England, France, and to a lesser degree US, Italy, and Germany) was a practice of the emerged capitalist states that started and flourished after the historically agreed upon dawn of capitalism (circa late medieval Italian city states and later the rest of Europe), and fed it.

Colonialism is theft. If they bought land from the natives that would be moral. Native people have all rights to home it has built, the land it has tilled and the livestock it has raised. Nothing to do with capitalism.