1. A compression method for compressing color images read by an optical sensor, comprising: counting the number of appearances of a hue value of each pixel of a color image, generating a hue histogram and storing the hue histogram in a storage unit; reducing the number of hue values of the entire color image, based on the hue histogram, allocating the number-reduced hue value to each pixel and classifying the pixels of the entire color image into clusters, based on the newly allocated hue value; and determining a characteristic of each cluster and encoding pixels in each cluster by a coding method suitable for the character of the cluster.