It seems like more earthy/camouflaging colours were suddenly used for uniforms and such things around the start of the 1900s. Why did they think the benefits of these uniforms suddenly outweighed the benefits of bright ones? What changed?
I guess I should say for the European, North American or any modern armies of the time. Did they change all at once or not?
Also, why were planes brightly coloured for much longer?
(Sorry if this has been asked before)
From my knowledge
Bright colored uniforms started to be phased out by some just before the First World War. Finally when the war started many nations still had their bright colored uniform, the French even with large elaborate head ornaments.
On the flip side the Germans wore a dull grey that blended in well to the fog and smoke and darkness of war.
Brightly colored uniforms worked fine with the firearms and projectile weapons were much less effective and accurate. War often was fought much closer, nearly hand to hand which is why the bayonet was so important (essentially a spear that could shoot) and sword were still wore as side arms. When fighting close quarters like that the colored suits were functional to identify friend from foe quickly and generals on the ground of from afar could identify movement and issue commands and formations.
The change came before the war started though. Machine good weaponry, firearms and other tools such as grenades improved as technology was applied. Suddenly the bright uniforms didn't help in close combat, instead they made very good targets from afar even in hazy conditions or at dark.
Many nations found their men were being shot down and forced to withdraw nearly as soon as the battle started, whereas forces such as the Germans appeared nearly invisible in the distance.
It didn't take long for nations. To rethink their ideas and change the uniforms.
Some other factors as well.
Before WWI army were relative small. Sometime during the neopolanic era and the wars that came after, nations learned to organize no only small regiments or mercenaries, but entire nations. For the first time ever armies of thousands, marching miles long were put into the field. This was a major shift in how war was waged and with the increase of technology also the body count.
Keeping an army fitted and clothed in the complex colorful and traditional garments became impractical and expensive.
Furthermore with the change in technology and the increase in military size maneuvers developed to be more complex and the arms of was were more difficult to carry as well. Simply put, the uniforms slowed down the military and became a burden .
All those factors combined made the shift to lighter, more durable, and better camouflaged uniforms am obvious choice.