Ceramic Crown color and cement?

It seems that your good dentist had chosen a more translucent porcelain material such as Emax crown to block out the underlying dark color, unfortunately translucent porcelain does not block out the discoloration, however they look more natural. If you wanted more white porcelain, then zerconia or lava crowns, they block out much more discoloration, not 100% but better than any other all ceramic crown material. If The discoloration is severe, the only crown that can block out 100% the discoloration of the prepared tooth is porcelain fused to gold!
Best of luck to you 

Hi:It is difficult to give you accurate advice without photo.  Please post a photo of your existing crowns.Best,Dr. Maddahi

Ask your dentist to show you the bleach shade guide. The bleach shade guide ranges from bl4 to bl1. Experienced cosmetic dentists will have the bleach shade guide, or your dentist can borrow it from the lab.