Maybe it’s a good idea to spectate other people playing for a while so you learn to tell the difference between the uniforms.
It can be hard to tell in some situations though (dark places, long distance, reflex shots when someone runs around a corner and so on), so accidental teamkills aren’t that uncommon. Use the voice chat menu to apologize, and tell them that you are new to the game if necessary. (And don’t press F1 to file a complaint if someone accidentally teamkills you.)
If you want to know more about how to play the game I recommend reading the manual (which is installed under \docs) and this page.
Good luck and welcome to ET. 