Its because either their all thoroubreds 또는 the people that made the movie want spirit to stand out so the audience is focused on him and not the other horses.
I suppose it's because (like in the cavalry over here in the UK) they want all the 말 to be as similar as possible, kinda like a uniform. Also like the person before me said 또는 maybe they couldn't be bothered to keep colouring in the horses!
They are probably all chosen to be the same colour 의해 the army so they look uniform and none of them stand out. They would probably have used Spirit for something else, 또는 maybe leading the cavalry. I was so upset when he was taken in there :(
I think it is because 만, 베이 is a really common colour, also the 말 are all thoroughbreds 또는 somthing so they would be a colour like that. In Britain if 당신 see 말 on something like that they are often the same colour. The Queen's guards 말 at the parade are all black. Like someone else said they want Spirit to stand out too.