green stool

Hi, We started on solid food like rice cereals for her just 3 or 4 weeks back, and she also breast feeds. We mix the rice cereal with the breast milk or the formula milk. we feed her with the rice cereal and breast milk in the morning and night and with the formula milk in the evening. when we go out and when my sister is not able to breast feed her we put her on formula milk. But she is not often put on foemula milk because she herself doesn't like it so much, we give her only under stringent circumstance. Is there any other information you need regarding my niece green stools????? Kindly please help me to find a solution for her problem.