Once a colony has gained independence should be able to choose to enslave thier colonists and make them work for free. There would be an enslave button in the colony management section that enslaves your colonists. Of course in order to enslave your colonists you will need to build a new slave managment headquarters where robots are used to force your colonists one by one into slavery(the time it takes to enslave each colonist is 5 sec). risks of enslaving colonists include being denounced by other colonies and a chance for a slave rebelion. If a rebelion occurs it will cuase major damage to your buildings and you colonists will be freed(if it suceeded). In order for a rebellion to be superseded you will need to have enough robot policemen to supress the uprising. Robot policemen are a new rover unit made by the robot factory. They can fight multiple rioters at once.
Too complicated, would make lag