I recall reading about the trousers on here before... here it is: https://www.reddit.com/r/AskHistorians/comments/3krtcu/why_did_the_french_wear_blue_uniforms_during_ww1/

But it only talks about how they had already decided to start switching uniforms before the war (only just), not how widespread the red trousers and/or the new uniforms were at the time of the outbreak.