A Flickr collection functions exactly like Flickr sets. Anybody who has the permission to view your photos can see those photos in a collection too. For example if there are some private photos in your collection, and no one has the permission to see them, then in that case nobody can see those photos. Until, you choose those private photos to be one of the mosaic photos. When a private photo is chosen as a mosaic, it is visible to all. However, they can not be seen within the collection.