Each gel syringe was re-labeled with a key number depending on the concentration of the agent, which was determined by an operator (G.Z.) who was unaware of the procedures. All color measurements were performed on the upper lateral incisors by different operators from those mentioned (C.A. and C.A.). In one group, the hemiarcade was bleached with 37.5% hydrogen peroxide (Polaoffice + 37.5% SDI Limited, Bayswater, Australia). In the other group, the hemiarcade was bleached using 6% hydrogen peroxide (Polaoffice + 6% SDI Limited, Bayswater, Australia).