Tooth turning grey is a sign of nerve death as  a result of trauma. There are chances that your tooth is already dead due to which you are not encountering any pain. You should visit your dentist and get an X ray done to see the damage inside. If the root has died than i suggest you to go for an RCT and then veneers.

For more info on you can visit the link...http://www.identalhub.com/articlecategory-stained-teeth-12.aspx