How can I remove a stain from a filling? (Photo)

I suggest to remove that old filling and when do the new one remove the stain and do a bisel in the tooth to mask the stain.

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 something you are insecure about then the most conservative way to address this would be letting your Dentist know and having the fillings replaced. The Dentist can replace your fillings with composite (filling material) that can be very natural and mimic the same shade as the rest of your tooth.

Some composite materials simply change color over time, some are more color stable. The photo does not appear to be a stain, but rather a filling that has changed color (assuming it matched when it was first placed). The only solution is replacing the filling.