Looking at the Osprey text in Fred's post and also at the other Haythornethwaite reference that I missed first time I looked
, it seems that like the heavy cavalry the buckskin breeches were part of the full dress uniform. When on campaign green overalls were often worn to protect the breeches and later perhaps the overalls were worn as trousers without the breeches.
But as I said light cavalrymen liked to cut a dash so perhaps the duty squadron escorting The Emporer might just like to wear their fancy breeches.
One thing to consider is the figures you are using. The Buckskin breeches were breeches, not trousers so if your figures are not wearing hessian boots then they should certainly be wearing green overalls. If they are wearing hessian boots then you have the choice of buckskin full dress breeches or green No.2 dress breeches.
For the Waterloo campaign I think I would favour green overalls. Actually I didn't really have a choice, my Guard Chasseurs a Cheval are from the Revell set and are definately wearing the green overalls with black leather strapping.