It was a longstanding tradition. Other countries had recently adopted camouflage uniforms, and in 1912, French Minister of War Adolphe Messimy saw their advantages and tried to change the French uniform. However, there was mass protest. The colorful uniforms, it was felt, were linked to Army prestige - which embodied national honor that had been besmirched by the loss of Alsace-Lorreine in the Franco-Prussian war and would someday be regained by military victory.

To banish "all that is colorful, all that gives the soldier his vivid aspect", wrote the Echo de Paris, a prominent newspaper, "is to go contrary both to French taste and military function." Finally, a former war minster cried in Parliamentary hearings, "Eliminate the red trousers? Never! Le pantalon rouge c'est la France! [The red pants are France!]"

The proposed change failed; French soldiers marched off to war with a "vivid aspect" that no longer performed any "military function."

(Source: Most prominently, Barbara Tuchmann, The Proud Tower and The Guns of August. William Shirer's The Collapse of the Third Republic also discusses this point.)

Basically, it was a new war with old tactics. Traditionally, militaries wore matching colors so they could be identified in the field. However, when world war one came along with more precise and devestating weaponry(Machine guns, Tanks, ect.), aircraft and trench warfare, it became better to be hidden than to be seen.

Also, an infantry column isn't going to help against a tank like it would against cavalry.