This is a very comple topic, in my view, only the Royal Guard as such did have "Royal" uniforms, the rest of the old Napoleonic Army did preseve more or less their dress.
Pierre Juhel did make a very interesting study - called

De l'île d'Elbe à Waterloo
La Garde Impérial en 1815

He found out very interesting information - looking onto the dress, equipment and uniform lists of the units, like e.g. Grenadiers à Cheval did still wear their old coats and not the new Royalist ones (as was assumed for a long time), as well as there were sufficient bearsking - caps available to equip all guard infantry of old and midde guard and so on.