The only difference is generally the main color, the defenders of ulthuan are the white and blue and the color that is usually changed is the blue, i think chrace uses red and nagarythe uses black.
Then it comes down to where your force is from on how many of certain units you would have, hoeth and saphery you will see more mages and sword masters, Averlorn and obviously chrace you will have more white lions and lion chariots etc, but thats only if you are basing an army on that area, take what you like in your list
But I think generally units change there armor color for the force/region they are fighting for.