Table 3. ∆E values (±SD) of lighter and darker teeth for each tested bleaching system. Means with same superscripts in a row within the same group are not significantly different at P<0.05 .

Before Bleaching/After Bleaching	Opalescence Oh	Treswhite Supreme	Hollywood Smiles
Lighter teeth (∆E1-LT)	14.8A±2.9	12.6A±2.9	3.3B±3.1
Darker teeth (∆E1-DT)	36.1a±8.1	29.1b±7.5	22.7c±4.0
After bleaching/After staining	Opalescence Oh	Treswhite Supreme	Hollywood Smiles
Lighter teeth (∆E2-LT)	11.1X±2.0	11.2X±2.0	9.4Y±2.1
Darker teeth (∆E2-DT)	13.8a±2.5	13.2a±4.8	8.3b±2.8

After bleaching, a significant increase in L* values and a significant decrease in a* and b* values were observed for Opalescence Oh and Treswhite Supreme in both lighter and darker teeth. Hollywood Smiles exhibited significant differences in L*, a*, and b* values in darker teeth only. The staining of samples produced a clear reduction in L* values and an increase in a* values for both lighter and darker teeth. After staining, no significant difference was observed in b* values for all the bleaching systems. For lighter teeth treated with Opalescence Oh, there was also no significant difference in a* values after staining.