Fillings, unfortunately, can't be whitened with the whitening strips you are using. The hydrogen peroxide would be ineffective on them, same as if you had any crowns. Fillings can change color over time with darkened foods and beverages like coffee, tea, red wine, soy sauce, etc. If this is...
If it bothers you aesthetically, you can change it. Also a gray reflection appears. You can indicate that attention should be paid to material selection.
