People who whiten teeth carrying existing restorations often find the result uneven. The restorations have not changed at all, and that is inherent to how bleaching works.
Bleaching needs organic pigment to act on
Peroxide breaks down coloured organic molecules held within enamel and dentine, splitting the bonds responsible for absorbing visible light.
Ceramic takes its colour from metal oxides fused into a glass matrix, and those oxides are chemically stable and unaffected by peroxide.
Composite resin holds pigment particles suspended in a set polymer, which is likewise not broken down by the reaction, so its shade stays fixed. Gold and other metals are unaffected for the same reason.
The mismatch appears only after treatment
A restoration is normally matched to the surrounding tooth at the time it is placed, so it is inconspicuous at that point.
When the natural teeth lighten and the restoration does not, the difference that was previously invisible becomes obvious.
Front teeth show this most clearly, and a single bonded repair on an incisor edge can become noticeable after a course of whitening. Older composite that has itself darkened slightly can end up looking darker still.
Sequencing avoids the problem
Where restorations are planned and whitening is wanted, whitening is normally completed first so the final shade is known before matching.
A settling period is usually allowed afterward, because teeth rebound slightly as they rehydrate and the shade recorded immediately after treatment is not the stable one.
Matching to that stable shade is what prevents a restoration that looks correct on the day and wrong a fortnight later.
Existing restorations may need replacing
Where restorations are already in place, achieving an even result generally means replacing them after whitening rather than trying to lighten them.
That has a cost and involves removing the existing material, which is a genuine consideration when several teeth carry large fillings or crowns.
Knowing what exists in the mouth before starting is therefore useful, and many people are unaware how many restorations they carry.
Peroxide can still affect the margins
Bleaching does not damage sound restorations, but it can temporarily reduce the strength of new bonds, which is why bonded work is usually delayed for a period afterward.
Restorations with worn or leaking margins allow peroxide to reach dentine directly, producing sharp sensitivity during treatment.
A pre-treatment examination identifies those margins, and sensitivity concentrated around one filling is a reason to be seen rather than to continue.