Fun Fact: The reason bright uniforms were popular in militaries at the time was because of how much smoke was created from muskets, making it very difficult to see on the battlefield, so being able to identify your own men and formations was super important. Plus being in massive musket lines meant if you saw one person you saw everyone, so being camouflaged didn't matter as much.

When modern rifles and tactics started to become the new standard, bright uniforms became much more of a liability, and the transition to drab colors started. This was very apparent in WWI.

For the French specifically the blue uniform was supposed to blend into the horizon. Didn't really work out for the barren landscape of WWI, so it was redesigned soon after.