As the tarpaulins were made from canvas I would assume they were a brownish / beige or grayish color already from their own. Just like the "civilian" tarpaulins.
Specially in the first part of the war, when camouflage was not standard at all.
Got this text from Wikipedia - Military Camouflage that can give some info (also on the 1910 uniforms!):
"French uniforms in the early stage of the First World War consisted of bright red (garance) trousers and blue Greatcoats as part of the standard uniform. An attempt to introduce a camouflage uniform in France in 1911 faced strong opposition as the red trousers were seen as a symbol of the French military doctrine. The former Minister of War Eugène Étienne stated:"Abolish red trousers? Never. France is red trousers." The red French kepi hats were however soon covered with cloths and the experiences of the modern warfare soon led to the introduction of a new uniform. The Belgian Army started using khaki uniforms in 1915.
The French established a Section de Camouflage (Camouflage Department) in 1915, briefly headed by Eugene Corbin and then by Lucien-Victor Guirand de Scévola. The experts were for the most part, painters, sculptors and theatre-set artists. Technological constraints meant patterned camouflage uniforms were not mass-produced during World War I. Each was hand-painted, and so they were restricted to snipers, forward artillery observers, and other exposed individuals. More effort was put into concealing equipment and structures. By mid-1915 the French section had four workshops (one in Paris and three nearer the front) mainly producing camouflage netting and painted canvas. Netting quickly moved from wire and fabric to raffia, burlap, and cocoa—natural materials were always recommended."