1. A color image region management system comprising: color class determination means for determining, for each picture element of image data in which color data of each picture element is expressed by a trichromatic system, to which color class the color of the picture element belongs of a plurality of color classes divided on the basis of a perceptual color system, and color index memory means for constructing tree structure data reaching from leaf nodes to a root node, each leaf node including at least one picture element, and calculating, for each node in said tree structure data and for each color class in regards to the picture elements included in the node, a frequency of picture elements which have a color belonging to the color class, and storing in said each node the calculated frequency of picture elements as a color index for each color class.