Nowadays, more and more patients presented to dermatologists complaining about "red face" accompanying with burning, stinging, itchy symptoms, or not. The red face is a straightforward clinical finding of facial erythema, and it is obvious even to the untrained eyes. 1 Interval flush or persistent facial erythema could be associated with multiple diseases, such as rosacea, contact dermatitis, acne, corticosteroid-dependent dermatitis, and systemic lupus erythematosus. 1 Hence, it is an important indicator in cosmetic science analysis and dermatologic diagnostics. In the past, methods of evaluating the facial erythema, such as IGA (Investigator Global Assessment) and PSA (Patient Self-Assessment), largely depended on the observers' personal experience and were prone to significant inter-observer variability, which could produce bias with subjective clinical scoring. 2 Therefore, more precise and objective methods and techniques of erythema assessment are demanded for the aim of evaluating and quantifying the erythema. With the rapid development of spectral imaging, image processing, and optical instruments, there have been developed varieties of skin color analysis methods or instruments, such as dermatoscopy, 3,4