The color quality of this piece is simply wonderful! I particular particularly love the way you rendered the red horse. And actually, you've done the people quite well.
The thing which you've done so well with the people is get the correct angle of the sloped shoulders and the legs. For example, the rider on the dark horse has her leg turned outward, just as she wouldn't real life. And you have her slumping in a very natural way. And the way the rider on the red horse has her head turned towards the other rider is very natural.
What you need to do to improve this drawing is one. The ground they stand on shouldn't end just as it touches their hooves. It makes them look as if they're standing on a knife edge and there's nothing behind them. Carry the background up and behind them somewhat, allowing it to fade off into the distance. Also, if you use a little bit of white for shine. What I mean, is for example, there would be a little bit of shine on the helmet top and on the horses rump, even the reins have a little bit of shine. Such use of light can really give vitality to a drawing.