If you are not using all the players for you map, you can change your unit's ownership to a leftover player (make sure he is your ally first), but retain your own colour.
For example, your map is only 4 players, therefore you are not using Brown. You are playing Red. Change your Unit to Brown and make Brown your ally, but retain the Red player colour of your unit. You can't control it whatsoever, but you can still heal it, click on it and treat it as if it is on your team.