1. A method of discriminating longitudinal melanonychia, comprising: representing, by a processor, a digital color image of nail apparatus melanoma as three-dimensional vectors, each composed of RGB parameter values of each imaged pixel, calculating, by a processor, a latitudinal variable θ i and a longitudinal variable φ i for each of the three-dimensional vectors in an RGB space, based on the following equations: ϕ i = cos - 1 ( R i R i 2 + G i 2 ) , ⁢ and ⁢ θ i = cos - 1 ( R i 2 + G i 2 R i 2 + G i 2 + B i 2 ) , where R i , G i , and B i indicate the RGB parameter values of a three-dimensional vector of pixel i; calculating, by a processor, a discrimination index according to a distribution of points defined by the latitudinal and longitudinal variables; and classifying, by a processor, the found discrimination index according to a threshold and discriminating whether the nail apparatus melanoma is malignant or benign, wherein the discrimination index DI is calculated according to the distribution of the points defined by the latitudinal and longitudinal variables, based on the following equation: DI = 1 N ⁢ ∑ i = 1 N ⁢ [ ( ϕ i - ϕ _ ) 2 + ( θ i - θ _ ) 2 ] , where N indicates a total number of points; φ indicates an average value of the longitudinal variables of N points; and θ indicates an average value of the latitudinal variables of N points, and wherein, when the discrimination index is smaller than the threshold, the nail apparatus melanoma is classified as benign, and when the discrimination index is greater than the threshold, the nail apparatus melanoma is classified as malignant.