Front tooth stained and discolored. What can I do? (photos)

Is it possible to whiten or correct the staining on my front middle tooth? It stains quicker than the rest. It's been filled. Would replacing the filling correct it? Is there a way to protect a filling so it doesn't stain again? My dentist x rayed the tooth and found it was not dead and did not have a new cavity. She said I would have to get an ~$900 cap to fix it, because the tooth underneath the filling is probably just yellower. Prior to the filling, this tooth was not discolored.