1. A method of skin tone optimization in a color gamut mapping system, which is implemented in an image processor for performing the steps of: (A) transforming an image signal from a predetermined color domain to an HSV color domain for generating an HSV image signal; (B) performing a skin tone optimization on the HSV image signal for generating an adjusted saturation gain; (C) performing a color enhancement on the HSV image signal according to the adjusted saturation gain and a color shift signal so as to generate a color enhancement signal; and (D) transforming the color enhancement signal from the HSV color domain to the predetermined color domain.