The officer is mounted on a black. Many blacks are a very dark brown. So I've used that as the highlight with an even lighter brown around the mouth and sheath.
As we have not yet accomplished my dream of living next to the stables, overlooking the horses as they graze. I use photos of t'internet as a handy reference. In the case of the trumpeter's mount, I used the one below, but allowed the light grey/white dapples to accentuate the highlights somewhat more than one the real horse. If enough people are interested and like my results, I'd do a specific post on how I paint horses sometime.
You'll note that I've also gone for darker tones than my norm on two of the troopers, based on this example of a faded black:
Once I've completed the brigade command stand, I'll post up a picture of the full regiment.