Everyone knows that oil and water do not mix. Yet when they do, what is known as an oil slick appears. An oil slick creates a colorful layer of oil that floats on the surface of water. This layer is highly reflective due to the light wave interference between oil, air, and water.
This undulating refractive index makes for shifting colors that move and change with light. You will see the same effect in Super Oil Slick—an extremely popular Thinking Putty color that is downright mesmerizing.