Support for multiple factions is coming, it's coming right next after I've got the jeep and bazookas up to a level I'm satisfied for a while.
I'll start by introducing a third faction with brown soldier skins, most likely with French names as the game seems to be fairly popular in France!
JackMayol can tell us more about how RWR is doing in Russia 
I'm going with brown color for the same reasons there are already green and grey, the color has some qualities of blending into the background and it's a familiar color people associate with soldier uniforms easily. While you might argue that it's not making it any easier for the player to spot the enemies comparing to e.g. red or blue, I think the choice of colors are justified for enhanced atmosphere -- on the line between realistic war simulation and team sports game, RWR is somewhere in the middle a tiny bit more towards the war side, so compromises need to be made (distinguishable skins, but not THAT distinguishable).
The factions in RWR don't really represent real countries as such, but to give it a little more separation between factions, the soldier names are based on the top 100 first & last names from real countries, namely USA and Germany currently. It's a minor detail really, but I personally find it a fun little thing.
When creating a new game, you'll be able to choose which factions are included in the battle. Later when content packages are coming, the content package provider i.e. map maker can define a default way to play a map, e.g. the way it's supposed to be played.