I have noticed a bug with colonies.
I have button to release a colony and when I click on it, it gave me a choice of 3 possible colonies. But when i choose one of them nothing hapens.
I can not release any of my cities on other continents and form a colony even I have that choice.

Could you please post a save?

This happens to me too. I've been told it's because I have 18 civs in the game.