We have different contact types including individuals and organisation and within the organsiation different types of organisations. But I cannot understand why some organsiation show as a blue building and some as a grey building.
Welcome to CiviCRM stackexchange. CiviCRM allows you to create contact sub types under each contact types. When adding new contact sub type the form allows you to define the image url which is used to display against contact. If you ignore this field at the time of saving or have no values than CiviCRM uses default image to show against the contact i.e Grey building. The blue color building is for Organization contact type i.e Organization contact having no sub type.
Hope this clarifies your question