Abstract: Based on the theory of color science, two methods of gray balance used for three to four color conversion are proposed in this paper. The basic relationship about gray balance is set up automatically by the process of learning from the sample data. Then the the two methods are compared with the traditional equality replacement method. Finally, an improved separation model based on GCR （gray component replacement） concept is proposed and is applied to four color ink jet printing. Experiments results indicate that the method presented in this paper has three advantages over other traditional methods: higher accuracy of approach, stronger learning ability, and less sample data that should be measured. The algorithm is adaptable to different printing conditions （type of ink, paper, etc .）.