It is imposible to describe the "uniform" of french pilot. Pilot could wear uniform of their original branch of the blue horizon uniform or a mixture of the two. for the details Guynemer is wearing a dark blue uniform of the enginer that was the "parent" corps of the aviator and ballon pilot. it was the standard uniform of those who joined as aviator now some coming from an other coprs were still wearing their original uniform (its the case of nungesser who was a hussard)
have look there I give a description of french uniform and there is a color picture
Alfred Heurtaux