There are a few issues here that mainly come down to misunderstanding of how tactics worked during the 19th century.

First, modern combat tactics revolve around a quality of weapon that is dependably accurate. A rifle generally will hit the target it is being aimed at with error being the user, a musket is an inaccurate and finicky weapon that doesn't always work because of the limitations of the technology.

The musket has several problems, first is the firing mechanism which depends on a piece of rock (flint) striking a piece of metal to make sparks. The flint can shatter in your face when hitting the metal but also it becomes dull and can contribute to misfires. Next there is a problem of gun powder which is not up to the quality of our current smokeless powder that wouldn't be invented till the later end of the 19th century. Next and most importantly is the problem of accuracy, due to the difficulty of rifling and reloading rifles, muskets were used that had bad accuracy at best, even under the best conditions a musket could hit a man sized target roughly 70-80% of the time, again under ideal conditions.

The real problem with the musket was the human aspect, particularly in reloading. A musket requires a charge in both the pan and the muzzle with the ball. Sometimes soldiers would have flashes in the pans, sometimes they might fire their ramrod, sometimes they might miss the musket barrel and pour the powder and/or ball in the grass or have the charge pour onto the ground because someone bumped you. Combine that with the fact that soldiers were trained not in marksmanship (which is rarely emphasized due to the limitations of the technology) and the focus on movement, a soldier fighting in the Napoleonic Era is very different than in the contemporary era.

Second, the tactics of the era are focused around the limitations of the weaponry. Because the weapons are inaccurate and limited, Napoleonic era combat was more focused on combined arms movement to ensure the defeat of the enemy. The French often focused more on the Steel (bayonet) rather than volley fire, combine this with quality cavalry and the best artillery in the world, French tactics were far more mobile and more focused on melee.

Also, the reason why soldiers were in line was because the concept was accuracy by volume, since accuracy wasn't guaranteed by the technology. It was the logical and effective way of making muskets (a much easier weapon to use than crossbows, bows, swords, spears, etc) important on the battlefield.

Now, to uniforms; uniforms were of all things a regimental/nationalistic identifier for soldiers. It was a method of identifying that the enemy in blue is French, the enemy in dark blue is Prussia, the guy in white is Austrian (Hungarian if he has blue pants), green for Russian, and Red for British. Smaller nations had more unique uniforms and before the Revolution, French & British uniforms were up to the personal whims of the regimental commander.

So, if you were to be camouflaged, then the you wouldn't have anyone to fight with (since you'll be an enemy to everyone), and it'll be for naught because once you fire, you'll have a giant plume of smoke hanging around you. Even if you run, you'll always have a giant plume of smoke when you fire.

Generally, this is all just how warfare worked in this time period, it doesn't make sense if we look at it from our time period, but it was what was most effective.

Patterned camouflage is relatively new for standard uniforms but khaki uniforms were issued by the British for camouflage purposes, first in 1848 in India. It rapidly became the standard field uniform in India and saw service in the Indian Mutiny and the various other colonial wars, including the Boer Wars, By 1900 it was the standard British field dress for overseas service and was also adopted by the armies of the various British colonies. It was used by the U.S. Army from 1898 during the Spanish American War.

TLDR Improvements in small arms, artillery and ammunition began to make the need for camouflage uniforms obvious from the middle of the 19th century. They were first introduced by the British and rapidly spread to other armies.

British Rifle Regiments (as written about in Bernard Cornwell's Sharpe series) did use green uniforms in Europe in the 1800s. They were armed with a Baker Rifle, which was able to hit targets out to a much greater distance than a smoothbore musket, and their light infantry tactics reflected that. Traditional uniforms had a bright shako, bright red coat and bright white pants; Rifle uniforms had a dark shako, green coat and dark pants; that seems a deliberate choice to camouflage, but I don't know.

Command and Control of an army depended on the general seeing troops, and giving orders. Camouflage could prevent a general from knowing where his units were, and prevent effective control.

Tactics and techniques depend on the technology, weather, communications and weapons being used. Modern tactics aren't better, they're just modern; updated for the newest change another army can field. 21st century tactics with 18th century weapons is like bringing baseball team in to play duck duck goose, and does not make sense. Tactics are defined by the tools and political purpose of a conflict.

"Ranger" divisions in the British North American colonies (in the 18th century) such as Rogers' Rangers would at times wear green uniforms as well as other forms of early camouflage. They also used tactics based on Native American warfare in am effort to combat the French and their Indian allies in the wilderness battles of the French and Indian war (North American part of the Seven years war). As a whole the ranger experiment did not prove to be too successful. During the American Revolution some of the Colonial militias would adopt sort of guerrilla tactics that were in part inspired by the rangers. So to answer your question more concisely, there were armies that used a sort of camouflage uniform, however it was not a very successful experiment so nobody ever really adopted the idea.