How can I whiten my porcelain crowns over implants? Also can I whiten veneers?

Hi:Normally the color of porcelain can not be changed, however if it is due to consuming staining liquids then you can try Oral Essentials whitening strip to remove the stain.Best,Dr. Maddahi

Hello and thank you for your great question.The great news is if your crowns and/or porcelain veneers were white from the start, they will be white again after a good cleaning with a hygienist. Porcelain does not change color internally. You most likely have a staining issue. How long ago was your last cleaning done? I bet it is time.

Hi, Thank you for submitting your question about whitening crowns and veneers. Unfortunately, and fortunately, unlike natural teeth, crowns and veneers are made from porcelain, zirconia, or ceramic. When we go to bleach our natural teeth, the peroxide in the different bleaching gels actually gets into the pores in the enamel (yes, enamel looks solid, but it has pores) and reacts with the organic material that is causing the stain. The bleaching gel will then create oxygen which breaks down the stain, and the teeth get whiter. I always compare dental bleaching to a good facial. When you are talking about crowns/veneers, porcelain has no pores, so there is nothing for the bleach to penetrate into. Now, the good part. Stain on crowns/veneers is always going to be external, or on the surface of the porcelain. For that reason, crowns/veneers will never change color/shade. Have you ever seen an older person that had crowns done 30+ years ago that look really white, while the natural teeth look yellow? Well now you know why.
So, what's the solution. The solution is a good dental cleaning and polishing by your dentist/hygienist. Once the superficial coffee/tea stain is removed, they will go right back to the shade they were before. As for whitening toothpastes, I recommend my patients to stay away from using them every day, but you can use them from time to time. I tell them to use whitening toothpastes on the weekend, and go back to your regular toothpaste during the week. The only toothpaste that I truly recommend against is something with baking soda. In my opinion, baking soda toothpastes are too abrasive, and long term will damage the surface of your teeth or restorations. 
I hope this helps you out, and best of luck,
-Dr. Alvarez