Press 0 to change colour of the horse. Press B to change background. It's simple, just drag the tack you've chosen to the right place on the horse.
Tacks and horse from Howrse. 23.10 - Added Bucephalus' tack. [day unkonown] - Added few more horse coats, "homemade", so please don't be mad at me if they don't look like "original".