In my macedonian game, I decided to let Epeiros alone and gave them lot of money. They conquered Arpi from Rome. By curiosity, i sent a spy to see what they decided to put as government, as there's no polis there. They built an allied democracy (fine) and... an hellenistic colony.
-They don't own a metropolis so they shouldn't be able to colonize. I didn"t see anything in campaign_scripts allowing AI to colonize without metropolis
-Colonies can't be built in allied states.

So, how does AI managed to build this ? Does the engine spawn a random building every X turns ? Is there a hidden script alloxing AI to colonize ?