What about real life military symbols. Star for a general, Chevron with one bar for a Marshal, Chevron with no bar for a vice Marshal.
Also the message section could be made to clear it up. When you're in an army instead of saying "Marshal of Army X" it could say "Your Commanding Officer", just like for lords it says "your liege".
As much as I love that idea, you have to remember that this isn't a modern military. It's a collection of landed nobles who hire their own private armies. It's up to the characters to bring any sense uniformity on themselves- its part of the challenge of working with others.
