It is hard to see how they could be mistaken. The Guards of Paris wore a distinctive green uniform, and at least the elite units wore the same sort of bearskin as the Chasseurs a Pied of the Imperial Guard. This according to the Otto Manuscript.
Contrary wise the Guard Fusiliers (also depicted in the Otto Manuscripts) wore a rather "common" blue uniform with shako...it would be almost impossible to distinguish them from line infantry at a distance.
A subject that isn't often discussed by us is the common misidentification of units in battle. For instance, Bennigsen repeatedly reports to the Tsar that he faced the French Imperial Guard, obviously mistaking elite companies of the line by their bearskins.