Why is one of veneers changing color after 3 months. It appears greyer and a shade darker

Based on your description, this could be due to nerve death within the tooth, leakage of between the tooth and veneer, or staining.  Staining usually would present on more than one tooth, so this cause is less likely.  Good luck and I hope this helps. Follow me if you have more questions.Sarah Thompson, DMDReal Self 100

The most common causes for discoloration to occur after veneers are placed is leakage or tooth color changes. A break in the adhesive seal of the veneer can allow leakage between the veneer and tooth surface. This allows fluid/stain to gather underneath and cause a darker color to be perceived. Another possible cause is that the underlying tooth is changing color and this darker shade is showing through the veneer. Since veneers are thinner at the gum line this is usually where you see leakage or tooth color changes. Go visit a dentist that is familiar with porcelain veneers and have them evaluate it. We use the Waterlase iPlus laser system to remove veneers so we can be very conservative. There is a possibility that if you find a dentist with the laser system they could remove the veneer and evaluate the problem without removing anymore tooth structure. We wish you the best of luck!