Before the advent of modern camouflage, military uniforms used to be solid, clearly identifiable colours. Battles were generally conducted in fairly good sport, and out in the open. Guns needed to be manually reloaded after each round, so there was very little smoke in the air, and most of the fighting was still done with blades or bare hands. To the practiced eye of a 'military merchant', it was quite easy to tell which way the tide of the battle was going.
We may now call them mercenaries, but lending martial skill for money is one of the oldest professions. These warriors were not conscripted, nor were they fighting for a cause. They wanted cold hard cash in their pockets, not cold hard steel in their lungs.
If it seemed that they were on the losing side of the battle, these level-headed businessmen would turn the heavy overcoats of their uniforms inside out, showing only the drab inner lining and concealing their erstwhile allegiances. They would then make their way quickly to the sidelines and stay well out of the way until the fighting was done, when the battlefield was ready to be scavenged for jewelry, money, and other valuables that had belonged to their late step-brothers in arms.