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