At the Start of World War I the french were resplendent in their new sky blue uniforms. It was said that they were sky blue so they would blend with the horizon. The reason is more likely one of supply. The original design was for a patriotic uniform using alternate red, white and blue threads that would result in a modern grey colour of uniform. However the red tread was to be supplied from Germany a bit of problem when you are going to war with them. Hence the uniform was made of blue and white thread making the striking sky blue uniform.
It night seem a little bright but look at the uniform it was replacing.
The German poster “Field Uniforms of our Enemies in the West” from 1914 shows the variety of colourful uniforms of the French, British and Belgian armies.
These uniform designs almost date back to Napoleonic times.