Grey lines on the teeth can be due to staining(intrinsic or extrinsic), cracks in the enamel or may indicate a dead tooth(if the colour progressively worsens).
Intrinsic stains were introduced during tooth formation. These can only be corrected by renovating the teeth, placing a covering over them to hide them.
Extrinsic or surface stains should be easily removed by your dentist.
Cracks in the surface of the tooth depends on the depth. Surface cracks are quite common and if the patient finds them ugly, you can also renovate them either by composite or porcelain veneers(there is a recorded webinar on this website about Dental Veneers that you may find useful). More serious cracks may require conventional fillings or crowns. Dead teeth may require either extractions or a root canal treatment followed by a crown.
