Purpose: To measure the influence of light activation, color pigments and extended application time on the in vitro bleaching efficacy of a hydrogen peroxide bleaching agent. Methods: Ninety-six 2.5 mm thick bovine enamel and dentin samples were selected for the study. Samples were stained in a tea solution for three days and then randomly divided into three groups. Bleaching was performed with a 35% hydrogen peroxide gel containing different color pigments. Red pigments were used in group one, black pigments were used in group two, and no pigments (transparent gel) in group three. In these three groups the bleaching agent was applied once every 15 min (4 x 15 min). Then each group was divided into subgroups: with or without light activation. In addition, a fourth group with red pigment was used to test the efficacy of an extended application time (2 x 30 min). All samples were measured with a spectrophotometer to determine the L*a* b* values before and after bleaching. The ΔE value was then calculated. Analysis of variance and Duncan post-hoc test were used to identify differences between groups at a preset a of 0.05. Results: When black and red pigments were used on enamel, light activation of the bleaching gel had a significant effect on the ΔE values. On dentin without light activation, transparent gel was significantly different from the red and black colored gel. There was no significant difference in the ΔE values between 4 applications of 15 min each and 2 applications of 30 min each. Conclusions: Within the limitations of this in vitro study, no difference in the bleaching effect could be observed between applying the product 2 times of 30 min each and applying the product 4 times of 15 min each. The effect of light activation on colored gel was limited to enamel surface.