Objectives: To determine in vitro the effect of prophylaxis before tooth whitening and to evaluate a paint-on home whitening product using two methods of color analysis. Method and Materials: Ninety extracted human maxillary anterior teeth were randomly separated into a prophylaxis group or nonprophylaxis group of 45 teeth. The prophylaxis group received debridement and polishing before whitening. The two groups were randomly separated into three subgroups of 15 teeth each: placebo group, test group (Easy White, Dental Kosmetik), and positive control group (Colgate Simply White, Colgate-Palmolive). The 2-week whitening treatment consisted of applying one of the three gels twice daily according to the manufacturers' instructions. In vitro measurements included tooth color assessment using digital imaging analysis and evaluation of tooth shade by a colorimeter. Measurements were taken at baseline, after prophylaxis, and after whitening. Results: While whitening was effective with or without prophylaxis, prior prophylaxis contributed to improved posttreatment outcomes. Both test gel and positive control gel resulted in greater shade reductions and tooth color improvements compared to placebo gel (P .05). The positive control gel was not superior to test gel. Conclusions: Prophylaxis is highly recommended before use of paint-on home whitening gels. Colorimetric measurements and digital imaging analysis with a gray card are options to record the efficacy of whitening products. Digital imaging analysis has advantages: Numeric data can be evaluated, and an image of the outcome of the whitening procedure is available. Keywords: colorimeter, color measurement, digital imaging analysis, extrinsic stain, gray card, prophylaxis, tooth color, tooth whitening