Color analysis was performed at the end of all treatments. The samples were placed on Teflon device (sample holder) inside a light cabin (GTI mini matcher MM1e; GTI Graphic Technology, Newburgh, NY, USA) to standardize the ambient light during the measurement process. The samples were submitted to color reading using a spectrophotometer (Konica Minolta CM-700d spectrophotometer Konica Minolta Investment, Shanghai, China) which was previously calibrated in accordance with to the manufacturer's instructions. The value of the without aging group without bleaching was considered as a baseline for comparison with the other groups.
The values obtained were quantified using the Commission Internationale de L’ E Clairage (CIE) lab system as three coordinates: L* (Luminosity, from 0 = black to 100 = white), a* (from axis - a = green to axis + a = red), and b* (from axis - b = blue to axis + b = yellow). These coordinates define the color of an object within a three-dimensional (3D) color space through the On-Color QC Lite software (Konica Minolta, Japan). Calculation of ΔE was obtained with the following formula: ΔE = ([L1 − L0]2+ [a1 − a0]2+ [b1 − b0]2)1/2.
The values obtained were quantified using the Commission Internationale de L’ E Clairage (CIE) lab system as three coordinates: L* (Luminosity, from 0 = black to 100 = white), a* (from axis - a = green to axis + a = red), and b* (from axis - b = blue to axis + b = yellow). These coordinates define the color of an object within a three-dimensional (3D) color space through the On-Color QC Lite software (Konica Minolta, Japan). Calculation of ΔE was obtained with the following formula: ΔE = ([L1 − L0]2+ [a1 − a0]2+ [b1 − b0]2)1/2.