AskHistorians

Sticking out like a sore thumb was the point - since troop formations had to be massive to concentrate the necessary firepower in an era before accurate or fast-firing firearms, there was often little interest in hiding those massive lines and columns. What was important, however, was figuring out who-was-who in the heat and smoke of battle - something made much easier if everyone is wearing easily identified colors!

As means of demonstration, in the US Civil War, when individual states were tasked with equipping and outfitting many units on both side of the war and had no set standards for what color or style to use, friendly fire was rampant because no one could tell who was on whose side. Hiding is less important than shooting the RIGHT people!

For this same reason, the American troops also largely wore bright colors in the American Revolutionary War - the main exceptions being militiamen, and skirmishers who were deployed ahead of the main force for harassing duties. The Brits themselves would adopt dark green for some of their specialized skirmishers shortly after the war.

The notion of obscuring troops instead of preventing friendly fire being a main purpose of the standard uniform is largely an invention of the late 19th century AD, when fast-firing single-shot rifles allowed even mediocre marksmen to pick off individual targets at 300+ yards. At that point, blurring one's figure, even just a little, becomes much more important. Into the 1st and 2nd World War, ever-faster and more accurate firearms and heavy weapons' support meant that combat increasingly became a game of hide-and-seek - he who is seen first, fucking dies. So camo is indispensable nowadays.