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Huevans is correct : the beige greatcoat worn by the French soldiers of the Napoleonic era was not officially introduced until April 25 1806, though some units had used it unofficially for some time.
Greatcoats were first issued to the Armée de Belgique in late 1792 after receiving reports of the hardships suffered during the Valmy campaign, which was fought during a miserably wet and chilly summer. However, supplies were limited and there were never enough greatcoats to clothe all the men, which meant many soldiers went without. Those that actually reached the army were often requisitioned civilian items rather than proper military coats, which meant they would have come in a variety of shapes and colours. Soldiers sometimes privately purchased (or looted) their own greatcoats, while some regiments occasionally pooled their financial resources to acquire some (again often civilian items).