by SW_Cygnus
Hey Jay,You are right that colonies can be a "forward base" for your Sea Captains. They allow your Sea Captains to winter there so you do not need to rebuild them next turn.
A colonies main purpose is to (hopefully) allow their owner, at the end of the card draw phase, to gain treasure and even a VP. For each colony, their owner will roll two dice, and check the result on the colony table. This is the only way a power other than Spain (or the power who controls Lisbon) can get treasures without diplomatic deals, or NW piracy. Check out section 8.4 for an explanation on gaining treasures in this phase.
You cannot launch new colonies from an existing colony. Your Sea Captains (not your colonies) are what you use to conduct piracy.
Steve
EDIT: Cleaned up my post, added more.