Let me ask you this Jon-What do you want to do?

If you are looking to put the officer on a typical appearing horse, then blacks or dark browns probably would be the way to go.

If you like that white horse though, just have a story to go with it. Many of the heavy horse breeds of NW Europe, the likely source for cuirassier horses, do include grays among the colorings. Perhaps the horse procurer settled for a healthy horse of the right size but wrong color (more likely after 1812 as horse losses needed to be made up) and the animal appealled to the officer. Perhaps the officer lost his own horse and this was the best replacement that could be found. Perhaps the officer used his own horse.