I like the idea of colonies as a sort of post-Project project for those nations who've already built all the available Projects. Maybe make a prerequisite for colonies being that you have built all the available Projects as only large nations could support a colony to start with.
You could build different colonies for different purposes:
a) A Resource colony would give you the mines (wells) worth of the max available for one city. Example: An OIL colony would give you resources equal to the output of 6 oil wells, since that's the max a city can make.
a MILITARY colony could increase the build cap on a specific class of troop. Example: A Foreign Air Base would increase your airplane cap by 5 air bases worth of planes, as that's the max a city could produce.
c) etc.
Finally, I don't want to sidestep the world market--colonies should be a back-up failsafe for getting resourcing when alliances power-pinch resources with inflated prices and market cornering. As such, I think that each colony should have a daily colony upkeep above and beyond normal production buildings (the cost of protecting, administering, and shipping in the goods to the motherland).
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