Watch what happens when liquids don't mix.
Water is denser, or heavier, than oil, so water sinks to the bottom of the glass. Water molecules are “polar” (more positively charged at one end and more negatively charged at the other), but oil molecules are not. Polar molecules are attracted to other polar molecules, but they don’t mix well with non-polar molecules. So the water and oil separate.
Like water, food coloring is polar. (In fact, it is made of food coloring molecules dissolved in water.) So the dense, watery food coloring drops do not mix with the oil. Instead, they fall to the bottom of the oil layer. When they push through the oil-water boundary, they mix with the water below.
Photo by: Guy Cali Associates, Inc.