The idea is very interesting, I do like the idea a lot.
But I guess there are two different ways of realizing the colony idea, espacially about the workers. Are the workers brought to the planet by ship, or are they natives?
A native population would be cool, for example a different species of Aliens depending on the kind of planet. Maybe there would be just a certain chance of spawning of native aliens on a planet. There could be trading features or simiular. Or with the colonization of the planet the player enslaves the native NPCs, making them his private workforce.
The alternative of bringing in NPCs as colonists is just as good, at least in my opinion, but where do they come from? It would be cool if they would be bought or rented or something in the shops, or maybe they can only be found at special places....
Either way, I like the idea of colonization a lot, it would add a whole new feature to the game making it very interesting to invest more time into planets.