Peroxide has no effect on ceramic or composite, so whitening with crowns already in your smile line only makes the mismatch obvious. The order matters.
Bleaching gel has no effect on ceramic, zirconium or composite. Peroxide works by oxidising pigment inside natural tooth structure, and a crown, a veneer or a white filling has no such pigment layer to attack, so it stays exactly the shade it was manufactured in. If you whiten with restorations already in your smile line, the natural teeth get lighter and the restorations suddenly look darker by comparison. Nothing has changed about them — the reference point moved.
Whiten first, wait, then restore. After bleaching, the shade continues to settle for roughly two weeks as residual oxygen leaves the enamel, so matching a crown on day two locks you into a colour that will not be there in a fortnight. Residual peroxide also weakens the bond strength of adhesive cements during that same period, which matters a great deal for veneers. We whiten, wait fourteen days, then take the shade and mill.
Composite fillings on front teeth are the common casualty. They were matched to your original shade years ago, they do not bleach, and after a successful whitening they read as grey patches near the biting edge. Plan for their replacement inside the same treatment sequence rather than discovering the problem in the mirror afterwards. Small front composites are inexpensive to redo; a full crown is not, and that changes the arithmetic of whether whitening is worth doing at all.
If you already have eight or ten ceramic units across the front of your upper jaw, whitening the remaining natural teeth usually creates a mismatch rather than a brighter smile, and we say so instead of taking the €180. The same applies in the days before a crown is fitted. Whitening earns its place when most of what shows when you smile is still your own enamel — otherwise the honest conversation is about replacing the restorations.
No. Its colour is fired into the ceramic and cannot be changed chemically. The only way to lighten a crown is to remake it, which is why the shade decision at the time of manufacture matters so much.
Fourteen days. That allows the shade to stabilise and residual oxygen to clear from the enamel, so the adhesive bond of the veneer is not compromised by the bleaching.