A crown returns from try-in and the patient complains it is too light. All of the following could explain the shade-selection error EXCEPT:

Correct answer: B. The shade was selected at the very beginning of the appointment

• A. Colour receptor fatigue is real: after roughly 5–7 seconds of staring the eye adapts and accuracy falls. Look at a blue-grey surface between comparisons.
• B. CORRECT — this is the EXCEPT, because taking the shade at the START of the appointment is CORRECT technique, not an error. The tooth is still hydrated, the eye is not yet fatigued, and the retina has not been bleached by the operatory light. Shade taken later, after rubber dam isolation and desiccation, reads falsely light — which is how crowns come back too white.
• C. Guide-to-guide mismatch (different manufacturer, different batch, or a worn guide) is a genuine and common source of error.
• D. Operatory lights are not colour-corrected. Shade should be taken under natural north daylight or a 5500 K colour-corrected source.

Note: Order of operations: shade first, before isolation, before anaesthesia if possible, with the tooth wet, at eye level, against a neutral grey background, in short glances.