Two teeth of the same shade can require completely different treatment, because the pigment causing the appearance sits in different places. The distinction shapes every whitening decision.
Extrinsic stain sits on an acquired film
Within minutes of cleaning, a tooth is coated in a thin protein layer drawn from saliva. Pigments bind to that layer rather than to enamel directly.
Coloured compounds from tea, coffee, red wine and tobacco accumulate on it, as do products from certain bacteria and some rinses containing chlorhexidine.
Because the pigment is on a removable film, this type of discolouration responds to professional cleaning and polishing without any chemical bleaching involved.
Intrinsic colour lies within the tooth
Most of what an observer reads as tooth colour comes from dentine showing through translucent enamel, so dentine shade largely determines the result.
Dentine becomes thicker and more yellow across a lifetime as secondary dentine is laid down, while enamel thins with wear and reveals more of it.
This is why teeth generally darken with age even in mouths with excellent hygiene and no dietary staining at all.
Some intrinsic changes have specific causes
Trauma to a tooth can cause blood breakdown products to enter the dentine tubules, producing a single darker tooth in an otherwise even arch.
Developmental disturbances during enamel formation can leave white, brown or mottled areas, and certain medications taken while teeth are forming produce banded discolouration.
These have different mechanisms and different management, and identifying which applies requires examination and often radiographs rather than shade comparison alone.
Why the distinction changes the treatment
Bleaching acts on pigment within the tooth, so it addresses intrinsic discolouration and does comparatively little for a heavy surface film.
Cleaning removes the film but has no effect on dentine colour, so a tooth that is intrinsically dark looks the same afterwards.
Attempting one when the other is needed produces disappointment, which is a common reason people conclude that whitening did not work for them. The product was often acting correctly on the wrong target.
Mixed presentations are the usual case
Most mouths carry both types at once, with a surface film sitting over dentine that has darkened gradually over the years.
Sequencing therefore matters, since cleaning first reveals the true underlying shade and gives a realistic starting point for any further discussion.
A single tooth that is noticeably darker than its neighbours, particularly after an injury, is a reason for examination rather than for whitening products.