Jazzo, you have to think back to the days before the invention of smokeless powder. Battles were fought by masses of troops arranged in units which advanced like chess men over the field in blocks of varying sizes. Battles usually began with an artillery barrage, which invariably left the field covered in dense smoke. European armies early on (late 17th century, I believe) adopted bright colored uniforms so the troops would be visible in the smoke which generally covered the entire battlefield. Different nations used different colors. The Brits adopted scarlet (a technical term for a particular shade of red), the French wore blue, the Austrians wore white, the Spanish, green. And the sense of professional pride made it hard to give up these colors when smokeless powder and more modern tactics made the colorful uniforms downright hazardous, as has been noted by others, notwithstanding there were lots of old veterans who hated to give up the red and thought khaki was the ugliest color ever invented (which is probably right, with the possible exception of OD [olive drab], which is the U.S. army combat uniform color, or was in my day).