As has been said, the country of an enemy is already displayed in the icon as the respective chess piece associated with that country (knight, rook, bishop.) While your idea has some merit, implimenting it would have adverse effects on the other uses of color in icons. Remember that Icon color not only identifies enemy (red) and friendly (green) but also signifies other associations like radio channel, mission, squad, wingman, etc. The colors selected for these secondary associations are customizable in settings and if you've ever played with them you quickly discover that red, green and yellow are by far the most readily distinguishable and legible colors. Other colors either lack contrast and blend in with the background or blur the text (like Black) making it hard to read.
However, the subject of your post brings up a complaint of my own regarding icons and that is that they are ridiculously large. At times during combat they all but obliterate the outline of the enemy plane coming at you. I know that they can be turned off, but that is a dangerous proposition too. My wish is that the plane icon and font were adjustable like the text buffer, allowing the player to decide how large or small, fat or narrow, bold or transparent he/she wanted that information to be displayed. Another possible solution would be to have it fade or shrink (but not completely) as the plane gets closer....say under 1k. Sometimes I find myself targeting the icon instead of the plane with unfortunate consequences. While the fat red icon helps identify the approaching speck in the sky as an enemy and helps me keep track of them in a fight, it really detracts from the realism...
Rise of Flight uses a very faint and thin font, admittedly sacrificing legibility but improving the visual experience and realism. It was one of the first things I noticed about that game and I've been meaning to suggest it.
