During the Feros: The Thorian mission, Shepard learns that ExoGeni found the Thorian and allowed it to control the colonists as part of a study. When returning from the ExoGeni building to destroy the Thorian, Shepard learns that under the control of the Thorian, the colonists will try to stop them. When Shepard arrives they encounter a Thorian Creeper on the exterior of the colony, once the creeper has been dealt with Shepard can make a choice, either to equip the gas grenades to incapacitate the colonists (paragon), or have the squad shoot them outright (renegade).
Colonists have low shielding and health, so they will tend to stay behind cover, usually with someone else. They will rarely leave cover and only take a few shots here and there. They are only armed with pistols so the damage they can put out is not much. Either the gas grenades, melee attacks or shooting them will be effective.