Copy-pasted from the 'GalCiv2manual.pdf' that came with the game, page 28:
Black Hammer: Colony has unused tiles and is not building.
Orange Hammer: Colony has unused tiles and is currently building a planetary improvement.
Yellow Ship: Colony has a starport and is not constructing a ship.
Green Ship: Colony has a starport and is currently constructing a ship.
Unhappy Citizen: This colony‘s residents are very unhappy.
Skull and Crossbones: Colony is in danger of revolting to another civilization.
Shield: Ships are currently orbiting this colony, keeping it from being invaded.
Agent: An enemy agent is causing trouble on this colony.