First you put milk and food colouring in a bowl. When you add soap all you see is a crazy collision of colours. But do you really know whats going on underneath?
When milk, food colouring and soap are combined the soap chases the fat molecules in the milk. This causes the food colouring to gat pushed and prodded which make the pretty colours that you see on top. Once all the tiny particles of soap have there own fat molecule the soap, fat molecules, proteins, natural sugars (from the milk), and food colouring will stop moving.
If you put more soap in this whole process will start all over again. Therefore the food colouring fireworks occur. So next time you do an experiment, try not to think you can see all that's going on.