Color analysis
The color of the samples was measured using a Spectrophotometer (PC-1, Shanghai Jia Bao Testing instrument Co., Ltd.) and reported using the Hunter Lab color scale. The scale pronounces L is the lightness, a is the red/green coordinate, and b is the yellow/blue coordinate. To evaluate the change in color between two measurement points, the total color difference parameter ΔE was used according to Eq. (3), as follows