Mileage will vary when it comes to pre-WWI camouflage. For a variety of reasons, in the 18th and early 19th century it was advantageous to outfit soldiers in bright, well-kept uniforms. It's especially true in the decades before the widespread adaptation of modern smokeless gunpowder. Black powder made so much smoke that it would give away your position anyway, and make it difficult to see on the battlefield. So it made sense for uniforms to stand out enough that soldiers knew who (not) to shoot at.

There are a few go-to examples for pre-WWI camouflage, but these are very much exceptions to the rule. The most commonly cited example you'll see is the use of the "Rifle Green" color on British riflemen starting in the Napoleonic wars. These troops were skirmishers, who were deployed well away from the main force to pick off officers, attempt to break troops from formation, and generally harass the enemy. It wasn't uncommon for these troops to take cover in forests or behind rocks, so their uniforms were designed with this in mind. In principle, they wouldn't be fighting side-by-side with the main formation, so they didn't need to worry about possible friendly fire.

Leading up to WWI, it was the Boer Wars in South Africa that saw the first widespread use of camouflage. The British found themselves fighting local guerrillas who were used to firing from behind cover and wore everyday clothing that naturally blended with the environment. The standard British uniform was a variation on the bright red jacket and blue pants that had been standard for over a century. This made them extremely vulnerable to the experienced Boer marksmen. The British quickly changed over to a solid khaki drab uniform, which mitigated the problem, although flaws in their core military doctrine still left them at a major disadvantage. Over the next few decades, the British standardized this uniform to their entire military, and most European countries followed suit with solid colored uniforms designed to blend in with their local environments. As I understand it, this is where camouflage technology was by the start of the First World War.

John Gooch's book of essays on the Boer Wars, "Direction, Experience, and Image" discuss the experience of the soldiers, among other things. It has some great passages on clothing and camouflage, and some discussion on why the British military doctrine put them at such a great disadvantage.