At the bottom right of the screen, you will see alien logos for each alien that the current unit can actively fire on. Most of the time, these are red, but occasionally they will turn yellow. What does this mean? Are there any other colors they can turn?
It means you just flanked an alien. If you notice the shield next to their name showing how much cover they are behind is also yellow. Also if you notice, when your soldiers are flanked, they have a yellow shield icon as well.
Thus far, I have only encountered yellow and red.