Color Changing Milk Experiment for Kids 

Explanation: Milk is mostly water, but it also contains tiny droplets of fat suspended in solution. Fats and proteins are sensitive to changes in the surrounding solution (the milk). The secret of the bursting colors is the chemistry of that tiny drop of soap, because of its bipolar characteristics (non-polar on one end and polar on the other), weakens the chemical bonds that hold the protein and fats in solution. The soap’s polar or hydrophilic (water-loving) end dissolves in water, and its hydrophobic (water-fearing) end attaches to a fat globule in the milk. 

This is when the fun begins! 

The molecules of fat roll and twist, in all directions as the soap molecules race around to join up with the fat molecules. As the soap becomes evenly mixed with the milk, the action slows down and eventually stops. 

Source: https://www.zlifeeducation.com/blog/color-changing-milk-3/