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