Lighter colors make rooms look bigger, darker colors make them look smaller.
Most people feel slightly claustrophobic and uncomfortable in medium or dark colored rooms.

soft pale celery or sage-y gray-green, cream or pale butter, sand, and very pale watery blue are serene colors. Stark white is abrupt in feeling, but can work well with color accents.
Red is very tricky, and can look simply awful.

Spare rooms are best not painted pink, lavender, or "boy blue". This is mentally limiting, strikes some people as too cutesy, and actually can work against you if people don't have children. Paint them as if the rooms are for guests or for home office or sewing room, etc. - light neutral serene colors. That way people can better envision them as multi-purpose rooms, thus seeing the house as larger and more versatile to their needs.