How about crowns and fillings?

Crowns and fillings will not whiten. Teeth whitening gels only work on your own teeth. If you have older crowns or fillings it is likely that these were shade matched to your teeth years ago. As years pass by the shade of crowns will not change making them look whiter than your own teeth over time. In this case you may do not have to worry. When you begin whitening your teeth will become brighter and their shade will be closer to the existing crowns and fillings making the difference even more visible.

If you have newer crowns or fillings or you want to whiten your teeth to a lighter shade then you may have to replace your crowns after you completed the whitening treatment.