In Indian culture guests are always considered as God. Therefore you should follow the principles of vastu for guest room also. It ensures the happy, peaceful and short stay of the guests at home.
Best location for guest room is Northwest direction this corner represents air element that is considered as the most unstable and movable element. Guest bedroom in this direction will ensure short and happy stay of guests at home. Other option is South or Southeast direction.
Avoid Southwest direction for guest as it is reserved for master bedroom. Guest will dominate or interfere if the guest bedroom is in Southwest direction.
Color of the walls can be white, cream, soft pink or light blue.
Windows in East and North direction is right option though it can be in West also but avoid South.
All the electric and electronic items should be in the Southeast direction, as it represents fire element.
South and West direction is ideal for cabinets and cupboards.
With a little care in the relative positioning and arrangement of the rooms one could expect to live a peaceful, happy & prosperous life.
Read about Master Bedroom Vastu here