The blue colour of french uniform is first because studies in the 1890s showed that the blue was harder to hit on different background. Then after the boer war some experiment were made, with different colours tested (green, different shade of blues, even very modern designs)
So when it was clear that something had to be done for the red outfit a shade of blue was chosen. There was an idea also that the uniform had to be unique compared to other countries.
During WW1, the french “African army” (spahis, chasseurs d’afrique, zouaves…) had “mustard” uniforms which were more or less kakhi.
French colonial troops in 1917-1918 started wearing “mustard” uniforms too.
After the war, some elements started being khaki, but the whole uniform stayed blue. In 1935-36 a new uniform was adopted with the “brown/kaki” color. But there were still huge supplies of blue horizons uniforms around.
Some second rate/supports regiments at the beginning of WW2 still had the blue uniforms.
A new uniform was supposed to be supplied in the middle of 1940 but only a few regiments had it in May 1940. But it just had minor differences to the 1936 ones.