Colonists are people in space-suits that are in need of rescuing. They are very small (in comparison to the ships you will be flying as Kyoto), and glow an unmistakable white/blue color from a distance.
They are a staple in this game, and nearly every mission in the game pushes you as the player to rescue as many colonists as possible.
If colonists are not rescued quickly, Manti Landers will swipe them up, and this in-game message from memory plays: "Colonist Captured". If Kyoto does not destroy the Manti that has captured the colonist quickly, the Manti will swallow him up and mutate, becoming a Manti Mutant.
Tip: If you are faced with the inevitable choice of allowing a colonist to get mutated, or die by falling (if you cannot pick them up), always let them die by falling, as the point penalty for allowing them to mutate is -600, whereas the penalty for letting them die by falling is only -50.