So, if you want unit colours to be the more than just one colour, how do you propse to tell the difference between teams in the campaign. Player has units, allied AI in the campaign, where applicable, will have units, need to  be able to figure them out. Also, there may be more than one team in the covenant in some levels, so you need to be able to see whats with what team. its a;ll a part of it being an RTS