Thanks Flambeau, you are absolutely right that people paint their figures to make them stand out. I'm less sure that the really bright colours reflect the reality. I'm guilty of this to some extent - not much mud on my soldiers' white trousers!

If you go with the "original" colours, say French or Prussian blue, you'll get the same effect that happened in reality at Waterloo and Ligny. When D'Erlon's and Bülow troops first appeared on the battlefield they threw friend and foe into confusion as from a distance their nationality could not be discerned. For some time Napoleon didn't know if D'Erlon's troops were French or Prussian and whether Bülows were Grouchy's or enemies. That's realism, but you may not want this much realism in a diorama, but rather keep the troops a little more discernable for the spectators. Details like dirty trousers actually don't matter for the more distant objects. If you paint them nevertheless it's for your own pleasure and the artistic expectation you have as painter, but unless a viewer is bringing opera glasses he or she won't see much detail of the more remote objects.
The problems come once you start taking pictures of your work, because then of course details again matter very much and bright colours suddenly may look rather too bright. So you have to decide what's more important.

You summarise the challenge nicely!