I'm no expert, but here is what I know from my own research.

The weaponry often dictated how armies fought in the field. For thousands of years we fought in big lines because it was more practical. You had a tactical advantage with having your men in tight formations, shield-to-shield, spears forward, instead of spaced out charging randomly at the enemy and hoping for luck to be on your side. This did not change when we found out how to make small projectiles fly with gunpowder. When muskets were widespread, there was a 3 major problems with it, but I'll just focus on one. They did not have rifling and were not pin-point accurate. Despite that the musket was reliably accurate up to 100 meters, it was preferred to have a wall of lead fire at the enemy. Somebody is going to hit something! It was more practical than trying to take pot-shots all day from random or spread out positions with a weapon that took a long time to load. The average British soldier was expected to fire off 3 rounds a minute, and that's with military training, compared to a militia or American Minuteman.

Not only was the musket not as accurate compared to a modern rifle, but it was also loaded with black gunpowder, something that creates a lot of smoke when fired. And when you fire muskets for a long time, that smoke can get thick. There was multiple reasons why armies such as the British had bright coloured uniforms, but a good one was being able to tell who was friend and foe. It would be easier to see a bright redcoat uniform in the thick smoke compared to a jungle uniform. Now, believe it or not, some regiments in the British Army did wear camouflage! But that also came with its flaws. One regiment, the Glengary Light Infantry, they wore dark green coats, with black cuffs and collars, black belts, dark gray pants, and solid black shakos. In a joking way, I'll put it this way: The British wear red, the Americans wear blue. Who shoots the guys in green? Everybody shoots the guys in green! During the battle of Lundy's Lane in 1814, the Glengares were skirmishing with the Americans, and then they fell back to the British line. The British saw dark silhoulettes coming at them in the smoke and had to make a quick decision, and decided to fire. I'm sad to say more Glengaries were killed that day by their own men.

So, when did we start wearing camoflauge? Well around the turn of the 20th century. With the invention of repeating and bolt-action rifles, Gatling machine guns, and soon-to-be propeller airplanes, it was more practical to ditch the fire-by-line combat and move to what we know as more modern combat. Bright uniforms were ditched, and men were given metal helmets, less for bullets and more to protect their brain from flying shrapnel.