Again - this plugin allows the player to choose his own skin. Replacing the default skin based on flags is not a part of it. In fact, I'm not sure how exactly that could be accomplished with the current setup. Maybe adding a "DEFAULT" option as a skin setting could do the trick.
The plugin doesn't modify how the skins look or work. If the skin is yellow, then the skin is indeed yellow. A plugin cannot change this in any way. Find a better skin. You might also have another skin with the same name in your CS folder, so the new one is not being downloaded.