When people say "royal guard," the first image that comes to mind for many is the British Royal Guard. These are the gentlemen outside the royal palace in the tall hats and red coats who must stand at attention for their entire shift. However, there are a lot of twists and turns you can take on the idea of a royal guard to make the costume your very own.
British Royal Guard
The traditional costume is the classic British Royal Guard. You need a red jacket, the tall, black furry hat and black military boots. The obvious tropes with this costume are standing at attention and not saying anything, playing a game with those attending a party to see if they can make you break character. You could also go around as if you were mute, making it a joke on the silence of the guards while they're on duty. Creativity can do a lot with this costume.
One simple costume change can make a lot of difference when it comes to your royal guard costume and this is to make it a clockwork soldier. The same jacket, trousers and hat are required, but you need to have some kind of large key like you would wind up a toy soldier with. The rest of the costume is acting; moving mechanically, using a monotone voice, and jerking when you begin to run low on clock spring energy.
Science Fiction Guard
If you want to do something a little different, take the basic royal guard and add science fiction touches. For instance, replace the standard issue rifle prop with a blaster rifle that would be more appropriate to "Star Wars." Use a set of pirate boots rather than military issue boots for a '50s sci-fi look, and add silver gloves to the costume. The rest will be acting, referring to events in the far future as if they were modern day, talking about Queen Elizabeth the 42nd and the new acquisition of colonies in space.
While going as a single royal guard is one valid idea, the costume would be greatly enhanced by getting others to go along with you. Soldiers travel in units, and if you have a partner, or even two or three partners it will enhance your costume. Especially if you all have the ability to walk in lockstep. The topping on the cake for this idea is if you have a king, queen or both that you all need to be guarding.