Not only were there different horizon blue colored uniforms, 1915 was a transitional year for the French army. They stopped wearing madder red trousers and started wearing all kinds of usually dull shades of grey, brown, khaki, blue, green trousers to replace them. These were often corduroy workmen's trousers. The top coat was not always horizon blue either. Some examples of 1915 clothing...
I'll add some more photos shortly...Have to use Photoshop to squeeze them to an acceptable size.
All the best,
Dan