Instructions
Turn on your Xbox with "Halo Wars" inside, and begin playing the game.
Select "Skirmish Mode" in the main screen.
Determine your color by choosing the number of teams in your game. The game automatically determines team color based upon the number of teams in your game from the same race. The 1st UNSC team is green, the 2nd UNSC team is yellow, the 3rd is blue, the 4th is red and the 5th is black.
For example, if you want to be blue, there must be two other UNSC teams in the game and you must make yourself the 3rd UNSC team.
Go to the Options menu from the main screen to change the color order for the teams. For example, you can make sure the first UNSC team is always blue, and so on.