I'm reviving this thread with French staff uniforms as per Ged's request.
French staff essentially fell into three categories : the general officers, the
corps d'état-major (staff officers proper) and the various ADC's attached to headquarters on an ad hoc basis. The first two were permanent corps whereas the ADC's were simply infantry or cavalry officers seconded from their parent unit for the duration of a campaign. Generals had an elaborate full dress uniform which I won't bother describing here since it was never worn in the field. Typical campaign dress for the generals consisted in a plain single-breasted black undress frock coat with an epaulette on each shoulder. Headgear was a kepi, occasionally worn with a white cover if the climate demanded it.
Corps d'état-major officers wore a full-dress coatee and bicorne for parades and ceremonial occasions, but field dress consisted of either the very elegant undress
spencer jacket (a braided dolman virtually identical to that worn by Chasseurs d'Afrique and Spahi officers, except that it was dark blue instead of light blue and red respectively), or the ubiquitous dark blue or black single-breasted frock coat worn with epaulettes on both shoulders and an aiguillette on the right shoulder. The trousers were red. Kepis were worn in the field instead of the bicorne, often with a white cover although sombreros could occasionally be seen as well. Most period paintings and photographs show staff officers in frock coats, albeit with a sprinkling of dark blue/black braided jackets here and there.
Beaucé's paintings occasionally show Spahi officers attached to Forey or Bazaine's staff as ADC's, as seen in the Assault on Puebla below or Forey's entry into Mexico City. This is intriguing as no Spahi squadrons served in Mexico, but since Bazaine came from the African army he may conceivably have attached a few spahi officers to his staff before sailing from France.
Most paintings show French staff dressed with few concessions to the Mexican climate apart from white covers and curtains on the kepis, and frock coats worn open to reveal a waistcoat and body sash underneath, though that may be because the actions depicted essentially take place in the cooler highlands. Bazaine did cut quite a dashing figure in his white half-cape and kepi cover : note his non-regulation jaguar-skin pistol holsters, which were supposed to make him appear as a modern-day reincarnation of an Aztec conqueror, the jaguar being associated with the Aztec emperors of legend.
Here is the photo Beaucé based his painting upon :
A
corps d'état-major officer in full dress :
A
corps d'état-major officer in undress spencer braided jacket (sorry for the poor picture) :
General Forey and his staff entering Mexico city. Note the spahi officer in red in the background.
Finally, here's a Roger Fenton photograph of General Bosquet in the Crimea wit his staff : two corps d'état-major officers in frock coats and two African army cavalry ADC's, one a spahi and the other a
chasseur d'Afrique.