Cement Showing Under Crown
What you are seeing is not cement. This is the underlying tooth structure that is showing through. At this point, the only way to correct this is to have the crown taken off and have more underlying tooth structure removed. This should take care of the problem and give you a much better aesthetic result. Best of luck.
Most all ceramic crowns will have a block out coping that will nullify the effects of shades of the glue. In those cases the issue is possibly inadequate reeducation of the tooth and the external layer of the porcelain is tooth thin and is showing the underlying coping. If the issue was the cement color, why wouldn't the other one look the same?Unfortunately the only thing you can do is talk to your dentist about it. Replacing the crown is your only option. When your dentist is cutting off the crown they should be able to look at the discolored area and determine if the issue was from thin porcelain or not.