Explanation: In the modern day, identification of friend or foe is a complicated process, considering that much of warfare is a game of "Who sees who first". There's a delicate balance of "Shoot first" and "Don't ice friendlies."
In the early modern period and into the 19th century (with khaki adopted by the Brits only near the end of the 19th century), increasing concern for friendly fire led to distinct and clearly colored uniforms being adopted by most modern polities, so as to identify friend from foe even in a smoke-covered battlefield. If you see red, fucking shoot!
Unfortunately, this was also undermined by how decentralized and near-feudal most states were at this time - many individual military units and military commanders insisted on unique uniforms or colors, deeply counterproductive to the core issue of "We need to figure out who is who in a very short period of time." Particularly, in the US Civil War, the innumerable state regiments, often aping the uniform of other well-known units like the French Zouaves, would often confuse each other for enemy forces and fire on each other, resulting in unnecessary casualties.
JUST WEAR THE DAMN STANDARD UNIFORM