A patient returns one week after crown cementation reporting the crown is too light. Each of the following could explain the shade selection error EXCEPT:

Correct answer: C. The shade was selected before tooth preparation

• A. This IS a legitimate cause. Shade should be evaluated under multiple light sources including natural north daylight, ideally at 5500 K color-corrected illumination, to avoid metamerism.
• B. This IS a legitimate cause. Shade guides differ between manufacturers and ceramic systems; a Vita Classical tab does not necessarily translate to the lab's material.
• C. CORRECT — this is the EXCEPTION, because it is CORRECT technique. Shade should be selected BEFORE preparation, at the start of the appointment, while the tooth is still hydrated. During preparation the tooth DEHYDRATES and becomes lighter and more opaque, so a shade taken afterward would be selected too LIGHT — the opposite of good practice.
• D. This IS a legitimate cause. Prolonged staring saturates the retinal cones; the eye should rest on a neutral blue-grey surface between comparisons, and each comparison should last only 5–7 seconds.

Note: Correct sequence: shade FIRST, tooth hydrated, eyes at tooth level, 5–7 second glances, multiple light sources, patient's lipstick removed.