Colonists are population units that determine the size of a colony.
The total number of resource slots in a colony determines the natural growth limit. By Annexing additional Sector and producing Buildings you can increase the number of Resource Slots. If by special circumstance a colony has more colonists then resource slots, the remaining colonists will become idle. Idle colonists cost upkeep, but do not generate any income.
Colonists have an upkeep cost of both Happiness and Food, so care must be taken not to grown a colony too fast, as that can lead to Rioters and Starvation.
Colonists have an upkeep cost of 2 Food and 1 Happiness. If the food income of the colony drops below 0 in the colonist growth pool a colonist will starve and be removed from the colony.
Through negative Happiness and hostile covert ops some colonists can become rioters. Rioters remain in their assigned resource slot but do not produce any upkeep and cannot be moved.
Rioters can be pacified by raising Happiness back to a positive value. For every 20 accumulated happiness one rioter will removed. Rioters can also be removed by imposing Martial Law at the cost of economic penalties.