You can't go wrong with light neutrals. There is a theory of colors and productivity. Every one that I have seen suggests light blue and gray near the top. Yellow is good for creativity. Red for doing more physical tasks but bad for sleeping. Green for balance.
Personally, I would go with something light Sherwin Williams Light French Gray or Sea Salt since the room will double as a guest room.
That's a very personal choice. I would go with a color you love, since you will be the one looking at it and living with it every day. I think neutrals are always nice, but if you generally like drama in your decorating and navy blue walls are calling to you, go for it!
Amanda: When in doubt --- ALWAYS go neutral. ...gray, beige, taupe, cream...
Do you have any fabrics that are in the room? That’s a great place to start for color inspiration. :)
Keep it neutral and restful. Avoid bright, bold colors, bold prints.
Place adequate lighting on dimmers to make it user friendly in both settings, and to avoid shadows.
Scandanavian design and color palettes keep things clean, calm, restful.