Dead (root-filled) teeth can change colour over time and look darker, greyish or pinkish. This makes them unsightly and noticeable in the smile, which is why many patients ask about having them whitened. A dead tooth can be whitened by applying whitening gel to its surface, but this is much less effective than whitening the other teeth in the mouth and takes much longer. Internal whitening of such problem teeth is also possible: the active ingredient is placed inside the tooth, in the space left after its filling has been removed. If the dead tooth does not respond well enough to whitening, other options for changing its colour are a veneer or a crown.
This answer is for general information. Your own case needs an examination by an orthodontist.