Just some observations, based on what I have read, to add to the above. I though Scott's version pretty accurate.
The Grande Armee in general was much more out then in regulation uniforms at Waterloo.
The mobilization...well to some degree actual 'creation' of units was very hastily done, in the 'Hundred Days'.
The Imperiale Garde even, wasn't even close to all being in regulation. For instance, the 3rd & 4th Grenadiers and Chasseurs a Pied, were mainly in Line uniforms and had their old shakos.
The eyewitness accounts of line infantry that come to my mind, often mention the infantry at Waterloo being in greatcoats, which I think were actually mostly a beige color. The white crossbelts would have to be worn over these. Because of the storm and rain on the 17th, it was a cold and chilly morning and mud was in widespread evidence on the soldiers.
I have been there twice at the same time of year and one was a perfect replay of storm and heavy rain the day and night before. By noon it was rather warm and increasingly so as the day wore on and I have always thought they had to be some hot and sweaty French infantry that afternoon if they were still in greatcoats. But maybe the pre-global warming temps were low on their day. Well now. " But Sire! To move my cannon, I must have at least 120 million hours to allow sufficient time for the ground to dry"

D).
The uniforms underneath the coats (probably incomplete) would have been (I think) 1813-14 era. When they say the French Army was mainly veterans at Waterloo, it was not all old-timers, but mainly the recruits of 1813-14, the 'Maria Louises' (spelling?). Any uniforms issued during the restoration would also have been a major component. The eagle and standard were new of course. They very likely had smaller red flags/banners, but I forget if this was for battalions or companies. One was mentioned on the ground at Waterloo in the area the 105th had been.
An odd thing then. Some at least, of the French line infantry regiments would not have appeared 'blue' at all, but more a faded faintly yellowish mass, with white lines. Like a regiment of spys in trenchcoats.