I don't play AgeII but I did beta test it. Initially you could attack and colonise with the restriction that the player you attacked was hostile (red).
It wasn't perfect but it made some sort of sense, as you could send attack waves to weaken them before colonising. The attack button was removed during beta testing and the game no longer made any sense to me. Perhaps I am overly aggressive but I thought Evony was a war game and not being able to attack any more changed it into something else.

Any way, the point I was trying to make is that there were both options (attack or colonise) originally, so saying that it would require more coding doesn't sound right to me.