What if players were unable to choose a color until they set their team number.
Then, each team has a set range of colors to choose from. So team one has dark red as an option but not light red. Team two has light red as an option but not dark red. This way each team could not choose very similar colours. You couldn't get light red and dark red on the same team.
If my team has a dark red player and the enemy has a light red player, my team will understand who I am talking about if i say "red go mid please" or if i say "about to snipe red".