The color subarea theory and the causes of error of color subarea theory were analyzed. The concepts of subarea boundary and sharing area and a modification method called the subarea boundary description were proposed. On the basis of this a judgment rule of whether the color is in sharing area was confirmed. By building multi-color separation model, the modification algorithm was verified. The effect of carrying out sharing area judgment or not were compared. The separation experiment result showed the subarea boundary description method can solve the error problem of color subarea theory, and can further improve accuracy of multi-color separation model.