From my experience, you get the blue Star as soon as you Buy Membership via credit card or buy a subscription via A cash from the shop. the Star dose not go away even if your membership expired or subscription expired. its there to show who is and who was premium.
but i have to agree Lydiarose, if your membership or subscription expired. remove the star or change the color. I don't care what kind of marketing scheme it is. It cause more problems then i think it worth. people are mean, as i state in other topic recently. free user are look down upon as lesser. i really hate it.
personally I all way post in my profile that my premium is gone and in courage other to so to.
as a side note, people really need to start reading profiles more.