Statement of the Problem: There is little agreement in the dental literature as to how much color difference constitutes an acceptable shade mismatch or how much color difference is perceptible to observers. Most studies attempting to determine perceptibility and acceptability of tolerances for shade mismatches have been conducted under in vitro conditions that are not applicable to clinical scenarios. Purpose: The goal of this study was to determine valid acceptability and perceptibility tolerances for shade mismatch in an actual clinical scenario using spectroradiometric instrumentation. Materials and Methods: A test denture was fabricated that allowed 10 maxillary left central incisors of varying shade mismatch with the right central incisor to be interchanged within the denture base. A spectroradiometer was used to determine the CIELAB coordinates and color differences (ΔE) between the right central incisor and the interchangeable left central incisor denture teeth. The interchangeable denture teeth ranged uniformly from 1 DE unit (visually undetectable) to >10 ΔE units (an obvious shade mismatch). The test denture with each of the interchangeable teeth was modeled by a subject to 28 dentists in a clinical setting. For each of the interchangeable teeth, dentist observers were asked if they could see a difference between the central incisors and, if so, whether the difference was acceptable. A Probit regression analysis was used to predict acceptability and perceptibility tolerances with 95% confidence limits. Results: The predicted color difference at which 50% of the dentist observers could perceive a color difference (50/50 perceptibility) was 2.6 ΔE units. The predicted color difference at which 50% of these observers would remake the restoration because of color mismatch (clinically unacceptable color match) was 5.5 ΔE. Acceptability and perceptibility color tolerances at the 50/50 level were significantly different. Conclusions: Tolerances for perceptibility were significantly lower than tolerances for acceptability for shade mismatch between two adjacent denture teeth in a clinical setting.