Although there is loads of evidence that man has used various things to disguise and camouflage throughout history it is interesting to note recent (last 100 or so years) of camouflage history today.
The French were the first to use camouflage as a protection of troops and equipment. That was WWI, 1915. Modern warfare was no longer only fought on the ground or at sea, but now involved aircraft and things could be seen from the air. That changed things big time.
France employed artists to paint equipment, clothing, and even soldiers themselves experimenting with various concealment options. Everyone else was quick to jump on the camo bandwagon. Camouflage patterns and techniques were developed and fine tuned.
Frog skin was one of the first American patterns developed.
World War II and each conflict after that further expanded patterns and techniques. They continue to develop to this day adopting new ideas and patterns as needed.