Objectives: the aim of this study was to evaluate the in vitro color stability of bleached teeth by means of a 6-day staining procedure. Materials and methods: 20 caries-free human mandibular incisors were randomly divided into two groups. Test group: a bleaching agent (Zoom 2, Discus Dental) was applied to the labial surfaces following the manufacturer's instructions. Control group: no bleaching treatment. Using a thermo-cycling device, all teeth were alternately immersed in a 0.9% saline solution at 37°C for 1 hour, and in a coffee solution at 55°C for another hour, in order to simulate the natural tooth-staining process. This procedure was repeated 12 times a day for 6 days. Teeth were polished once a day with a rubber cup and polishing paste. CIELAB color variables (L, a, b) were assessed by means of a spectrophotometer (VITA Easyshade®) and a custom alignment device at baseline, after bleaching, and once a day throughout the 6-day staining treatment. The bleaching result and the color stability of bleached and control group teeth were assessed by matched pair t test and MANOVA. Results: the bleaching agent had a significant whitening effect. The staining treatment did not produce any significant color change on bleached teeth, which maintained the same whiteness achieved after the bleaching treatment. The staining treatment did not produce any significant color change on control teeth. The two groups showed significant differences in both whiteness and lightness changes as a result of the polishing treatment. Conclusions: the Zoom 2 whitening agent produced immediate and long-lasting bleaching results, even if the reduction of b value mainly occurred 24 and 48 hours after the bleaching treatment.