Under normal circumstances I would say that times in which Regiments could pick certain colors for Horses from the available "Pool" is in Peacetime's. Rarely at War. Sure if you bread only, say, 3 Colors it might work to a point...but I doubt it.
Variations of White or grey and brown would start right after the first losses.
As so many things in War of this Times developed after the French Fashion white Horses, even in Black Mounted Regiments were used for Trumpets/Musicians.
There is also the Case - I draw this from a Napoleonic Saxon Regiment - that only the first Squadron was mounted on Black Horses.
But this Information left People who saw those Troopers on their large "Black" Horses to believe the whole Regiment had Black Horses.
(Most of the Horses were of a very Dark Brown).
by GÜNTER HEIM