Colonial Administrators are an essential part of any team. They are able to turn dust bowls into Edens. High level Colonial Administrators can easily get several multi-billion population colonies because of the increased suitability of their colonies.
Note that this bonus applies to the base suitability, not the increased suitability from terraforming. Also note that to apply the bonus, the character with the skill must own a base on the planet with the colony, and having more than one base on the planet does not affect the bonus given.
See also: Trade Skill Level Table