Abstract: The method proposed by the present invention divide an image into a plurality of cluster regions, each has its own principle color. The method mentioned above includes the following steps. At first, acquire a three dimensional histogram of the image by counting the number of pixels of the image based on three dimensional color coordinate. Then smooth the three dimensional histogram by a numerical method to obtain a function. Next, define a plurality of cluster regions on the image. Subsequently, assign the color of each pixel of the image as a principle color of a first cluster region among the plurality of cluster regions. The principle color is equal to average of all pixels within the cluster region, the average is equal to A/B, wherein A is summation of color of pixels within the first cluster region, and B is equal to the number of pixels within the first cluster region. Each of the plurality of cluster regions has a corresponding principle color.