You will need:
Instructions1. Pour some milk into a saucer (ignoring your cat: this is for the cause of science).
2. Drop four drops of different colours near the edge.
3. In the centre, add one drop of dishwashing detergent.
The drop of detergent reduces the surface tension at the point where it dissolves the fat molecules in the milk. The higher surface tension of the surrounding milk pulls the surface away from that spot, dragging the food dye with it.
The rest is due to the detergent breaking down the fats within the milk. As the fats break down, water carries the food colouring to where the fat was to fill the space.