Fig. 1: Skin color distribution of participants.

a Three-dimensional distribution of quantitatively assessed skin color indices in the CIE LAB color space. Each dot corresponds to a study participant and its color represents the measured skin color for that person. The diagonal plane represents the regression plane for “L* ~ a* + b*”. b Distribution of L* (top), a* (middle), and b* (bottom). Histogram shows the frequency of each skin color trait, and dotted line represents the cumulative density. c Distribution of categorical skin color classified by individual typology angle (ITA°) value: ITA° = [ArcTan((L* − 50)/b*)] × (180/π). Each dot corresponds to a study participant and its color represents an average value of the measured skin color of both cheek areas for that person. Blue dotted line represents a horizontal line at L* = 50 and black dashed line represents the ITA° cutoff for categorical skin color.