Adjustable Density Expanding Foam

This may or may not be exactly the same expanding foam that we use in our ASM teacher summer camps, but that graphic up there (something I found in casually searching for 'expanding foam' images - as you do donchaknow) shows something that we've noticed and only casually researched. Adding more food coloring (which comes in water, natch) isn't always productive toward what you want to achieve. In fact, adding way too much food coloring can cause the unexpected effect of less expansion as the bubbles get too big to support the structure and weight of the foam.

Check out Public Missles Ltd's article detailing this effect on their foam. It's the second page of the pdf that describes the effect of adding more water to their foam and why you might want the different densities that result.