Who wins if the game ends with one player having 6 colonies and 1 or more players with 5 colonies?
I received the following e-mailed answer from Jonathan Bove of FFG:


  Since all of the players fulfilled the victory condition at the same
  time, it is a joint victory. However, this shouldn't stop the player
  with 6 colonies from bragging about his or her intergalactic skills!


Since this is direct from FFG, I will take this to be the official answer.