I paint my miniatures based on the Battles of the Frontiers basically. The model 1913 simplified frock seems to have been common at the outbreak of war in August 1914.
I came across this photo showing the m/1913 frock belonging to a sergeant along with matching trousers dated 1912.
Now, did some junior officers of the line infantry wear privately purchased trousers in a matching iron-blue colour during the opening battles? Besides those wearing greatcoats with unbuttoned folds to conseal the red trousers which was an early measure to reduce visibility among officers and the men alike.
With officers dressing uniforms at their own expense and having the option of having their uniforms custom made by civilian tailors it doesn't seem unlikely. Or would it be more suited after Marne in late September when the French army started issuing drab overalls.
Just considering to maybe paint a few that way along with the usual madder
Thank You.