The French formed for example provisional Regiments of horse like 1st – 3rd Cuirassiers . Formed from escadrons drawn away from regular units. The 1st later became the 13th Cuirassier. At first the men wore the uniforms from the one they came from. Only at next replacement they got new dress.
Same with 1st – th Prov. Drag.
Same with infantry, most are formed around 1808 and most had become regular units in 1812 like 1st and 2nd Prov that became 114 de Ligne.
OOB was regular style.
In Russia most units wore a mix of regular dress and replacements from many sources. Even before campaign a lot was needed, a foot unit coming from Spain needed 2 -3 new sets of footwear on the way. Depots in Eastern France or Germany had rarely exactly fitting regulation stuff. So we find “wrong” buttons at camps from units that had never been there. Lost by men from other regiments wearing dress from the depots.
So if someone tells you your unit has a wrong shade at the cuffs give him a smile. Worse, the UV change. A unit that marched for weeks changed from dark blue to a mid one. Only if the men wore the coat most of the time the uniform stayed. A lot was not water proof. So light blue white belts are not wrong. 😉
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