Fanfan's white trousers get dirty from falling off a horse, but in the next scene they are clean.
This is a fun film not to be taken as history, but it is supposed to be set in the Seven Years War in the 18th Century. The troops are carrying percussion cap rifles from the 1860s.
Princess Henriette of France died in 1752, four years before the start of the seven years war (1756-1763). This movie takes place during this conflict and the princess should therefore not appear in this story.