I believe that the white "greatcoats" were actually linen "dusters" worn over the waistcoat and instead of the habite longue , during the early part at least of the invasion of Spain and Portugal; they were reported by British observers at Vimiero, I believe. If you have the book Military Dress of the Peninsula War by Windrow and Embleton, there is an illustration of a fusilier wearing one. I'm just painting up an SP2 force of French infantry (a mix of Perry and Warlord plastics, plus Perry metal command and characters) in a similar fashion, in bleached bone with off-white highlights and trying to make the material look thinner than greatcoats!