After reading the book Mouse Paint, discuss how primary colors mixed together to make secondary colors.
Give students 3 cups each with a scoop of white frosting. Circulate the room and add one drop of red food coloring in one of their cups. This will be their red "mouse paint." Continue with blue and yellow. Students stir the frosting till it turns red, blue and yellow. Then have them put a drop of red and yellow frosting on their first graham cracker and mix it together. "Look, orange." Do the same with blue and yellow, red and blue. Students can enjoy the eat the crackers at the end of the lesson.