When you started decorating your house you probably had very definite ideas about what you wanted to do in each of the rooms. Now, having successfully stamped your personality on every room in the house, you are at a bit of a loss as to what to do in the spare room. After all, this is where you are going to put your visitors - everyone from your brother-in-law to your grandparents will probably, at some point, stay in this room. How do you make sure it is welcoming to everyone?
The short answer is, well, don't! It is impossible to please everybody. Everyone has a different character and different ideas about how they want their bedroom to look, and this isn't actually their bedroom, it is nothing more or less than temporary accommodation. So don't worry too much about making it the perfect room for every single person who might possibly, at some point, spend the night there, and think more about how to make sure they will all at least be able to relax in it.
This means trying not to make any strong statements. You want people to get to this room and immediately be able to relax, so it would be a good idea right now to forget about doing the entire room in crimson or black, lime green, scarlet... You get the idea. Stay reasonably neutral, or completely neutral. You can do a lot worse than do the entire room in white, although it might be rather like waking up in an operating theatre.
However, you can decorate the room in white and then decide on one color you would like to use as an accent color. For instance, blue is a fairly non-risk color, so you could hang a couple of pictures with blue and white frames on the walls and put two blue pillows on top of the white ones already there. Blue candles on a white dresser will stand out nicely without assaulting the eyes. Similarly non-offensive colors include a deep green, darker rather than lighter, red or purple. Even orange and yellow would be good choices, provided you don't go over the top. The idea is to add a few splashes of color, not make the place look like an ice cream parlor.
Area rugs are an excellent way of adding color accents. A rug colored mainly white could feature your chosen accent color in small patches or as part of a geometric pattern. Avoid having an area rug entirely the same color as your accent color. That is not an accent. It is a statement and possibly something you want to avoid if you want everyone to feel reasonably at home in this room.
The important thing when decorating a visitors' room is not to go overboard on what you see as creature comforts. You may not be able to sleep without being watched over by several laughing Buddha statuettes but the chances are your guests don't feel the same way. Keep it simple rather than homely and your guests will definitely thank you for it.