Would the Crest 3D white strips could make the crowns get back the color it was (brighter)?

Porcelain veneers and crowns have a great advantage of being very stain resistant, however; this strength can be a weakness for some that do stain their teeth through heavy smoking or heavy consumption of curry-rich foods or dark wines and juices.  In other words, they are very hard to stain, but once they're stained, they can be very hard to un-stain.  Good luck and I hope this helps. Follow me if you have more questions.Sarah Thompson, DMD

Unfortunately crowns that are made with porcelain will not whiten with anything.  You can get off stain but using a whitening toothpaste like SuperSmile Toothpaste will help maintain the condition of your crowns even better.  It is a bleaching toothpaste and great to keep dental care whiter and brighter.  Good luck! 

It is very unlikely that a Crest White Strip will change the color of a the porcelain crown.  You could give it a try but I have never heard of it working.  Perhaps you will get lucky.  It's worth a try.  Good luck.  TR