Napoleon rode around 150 horses in his military career, including several bays and chestnuts. Of the five versions of David's painting, two portray him on a piebald (black and white) horse. There is plenty of scope for other colours other than grey.
The wild, flowing mane and tail are really an artistic device, looking very impressive in a painting, but very impractical in the field.
While Napoleon rode Marengo for part of the battle of Marengo (hence the name) the horse he rode crossing the pass was probably Styrie, but for most of the crossing, he probably rode a mule, as in Delaroche's painting.
We must remember that a commander would ride several horses in the course of a day, and particularly a day of battle.
Wellington did ride Copenhagen throughout the day of Waterloo, but this is certainly an exception.
Ed
