Its because either their all thoroubreds या 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 घोड़े to be as similar as possible, kinda like a uniform. Also like the person before me कहा या maybe they couldn't be bothered to keep colouring in the horses!
They are probably all chosen to be the same colour द्वारा the army so they look uniform and none of them stand out. They would probably have used Spirit for something else, या maybe leading the cavalry. I was so upset when he was taken in there :(
I think it is because खाड़ी, बे is a really common colour, also the घोड़े are all thoroughbreds या somthing so they would be a colour like that. In Britain if आप see घोड़े on something like that they are often the same colour. The Queen's guards घोड़े at the parade are all black. Like someone else कहा they want Spirit to stand out too.
