Instructions for the "Color Changing Milk" center activity.
Optional: Check out this second version by Doctor Mad Science!
Why does this happen? Dish soap is "bipolar" (nonpolar at one end and polar on the other) and weakens chemical bonds holding proteins and fats in the milk. Soap molecules join with fat molecules in the milk. Milk with more fat content produces more colorful results because there is are more fat molecules to combine with the soap.
This video provides a partial explanation of why the food coloring, soap and milk change as they do in this experiment. (Thanks @sfryer!)