A skin undertone determining method, comprising: obtaining a first image; obtaining a first parameter value and a second parameter value in a first color space according to the first image, wherein the first color space is a HSV color space, the first parameter value is a hue in the HSV color space, and the second parameter value is a luminance value in the HSV color space; obtaining a third parameter value in a second color space according to the first image, wherein the second color space is a RGB color space, and the third parameter value is a blue value in the RGB color space; and determining a skin undertone of skin in the first image according to the first parameter value, the second parameter value, and the third parameter value; wherein the step of determining the skin undertone of skin in the first image according to the first parameter value, the second parameter value, and the third parameter value comprises: determining the skin undertone of skin in the first image has warm undertones when the first parameter value is greater than a first predetermined threshold, the second parameter value is greater than a second predetermined threshold, the second parameter value is less than a third predetermined threshold, and the third parameter value is less than a fourth predetermined threshold; and determining the skin undertone of skin in the first image has cool undertones when the first parameter value is less than the first predetermined threshold, the second parameter value is greater than a fifth predetermined threshold, the second parameter value is less than a sixth predetermined threshold, and the third parameter value is greater than a fourth predetermined threshold. obtaining a first image; obtaining a first parameter value and a second parameter value in a first color space according to the first image, wherein the first color space is a HSV color space, the first parameter value is a hue in the HSV color space, and the second parameter value is a luminance value in the HSV color space; obtaining a third parameter value in a second color space according to the first image, wherein the second color space is a RGB color space, and the third parameter value is a blue value in the RGB color space; and determining a skin undertone of skin in the first image according to the first parameter value, the second parameter value, and the third parameter value; wherein the step of determining the skin undertone of skin in the first image according to the first parameter value, the second parameter value, and the third parameter value comprises: determining the skin undertone of skin in the first image has warm undertones when the first parameter value is greater than a first predetermined threshold, the second parameter value is greater than a second predetermined threshold, the second parameter value is less than a third predetermined threshold, and the third parameter value is less than a fourth predetermined threshold; and determining the skin undertone of skin in the first image has cool undertones when the first parameter value is less than the first predetermined threshold, the second parameter value is greater than a fifth predetermined threshold, the second parameter value is less than a sixth predetermined threshold, and the third parameter value is greater than a fourth predetermined threshold. determining the skin undertone of skin in the first image has warm undertones when the first parameter value is greater than a first predetermined threshold, the second parameter value is greater than a second predetermined threshold, the second parameter value is less than a third predetermined threshold, and the third parameter value is less than a fourth predetermined threshold; and determining the skin undertone of skin in the first image has cool undertones when the first parameter value is less than the first predetermined threshold, the second parameter value is greater than a fifth predetermined threshold, the second parameter value is less than a sixth predetermined threshold, and the third parameter value is greater than a fourth predetermined threshold.