Here is some video of when I taught the three color density lab while in the Philippines. I did this with over 100 students. To change the density of the water, we added different amounts of salt to create different densities and gave each layer a different color using food coloring. We then decanted each layer into a plastic cup. We did this outside the school under an overhang on a very hot day. As you can see, many of the students did very well.