When playing with friends show their country color as a 2 color stripe pattern - their color and a secondary color like black. That way you know you are playing with someone you've played with before and friended them. Right now, unless you choose a private match you can't tell if you've friended this person before and should adjust your behavior (i.e. my friends have a high trust factor).
When playing with players that you have an in game alliance with show their player color as a 2 color stripe pattern - their color and yours. That way it ups the visual appearance of friend vs foe on your screen. Sometimes I forget who I'm allied with and this would help.
This is an interesting idea. Perhaps all you would need is additional indicators in the player menu.