Why are my veneers turning yellow?

True Veneers which are .5 to .7 millimeter can improve and whiten the original underlying color 25 to 30%. Basically a true porcelain veneer can improve the underlying dentin, just as the enamel blocks the yellow color of the underlying dentin. If you had thin veneers and not regular porcelain veneer ( which there is more thickness to your porcelain) then based on dietary habit, and acidity in your mouth, there is possibility of color shift!  Also since you had a combined restorations, meaning crowns and veneers, if the porcelain material was not the same that can cause color discrepancy! for example a zerconia crown combined with emax veneers or regular porcelain veneers, it is very hard to get the color exactly right.More probability can exist as far as shift in color, if you do make an appointment with an expert cosmetic dentist 

it is difficult to say anything without seeing yr pics , so its my request to upload pics. and also mention veneer material. change in shade is common issue with composite veneer.

hi:Unfortunately without photos it is difficult to give you accurate advice . Please post photos of the veneers in question.Best,Dr. Maddahi