AbstractThis clinical trial investigated the effects of pre‐application enamel moistening on the impact of a 37% carbamide peroxide whitener on tooth color changes and the influence of repositioning guide colors. Forty participants were randomly assigned to in‐office tooth bleaching with either moistened enamel (experimental) or dry enamel (control). The whitener was applied for 45 min over two sessions. Tooth color was visually measured or assessed using a spectrophotometer with purple or green silicone guides. Tooth bleaching was assessed using CIE76 (ΔE_ab) and CIEDE2000 (ΔE₀₀) formulas and by whitening and bleaching index score changes. Moistening the enamel did not significantly affect tooth color. However, the guide color choice only impacted tooth color when measured instrumentally. At baseline, the green guide resulted in statistically significantly whiter teeth than the purple guide. Less pronounced differences in the b* coordinate between baseline and final measurements were found using the green guide. The green guide also produced lower ΔE_ab values and less change in indexes. In conclusion, moistening the enamel did not significantly impact tooth color changes. However, the repositioning guide color influenced the tooth bleaching measured instrumentally, except for ΔE₀₀.