It is possible to end a game of Cosmic Encounter with 6 colonies (i.e. Parasite wild flare). The rules for the FFG version of Cosmic Encounter state that:
The winner(s) are the first player(s) to have five colonies on any planets outside his or her home system . . . It is possible, through alliances and successful negotiations, to have more than one player gain five colonies at the same time. In this case, the players share a win.
This could be interpreted to mean 5 or more colonies, but the rules do not specifically say that. A devious tactic could be to invite someone along as ally to each achieve a 5th planet simultaneously, but then because the parasite flare was played the attacker ends with 6 colonies vs. the ally's 5 colonies.
So . . . who wins if the game ends with one player having 6 colonies and 1 or more players with 5 colonies?