There is a much simpler way to get the effect, as expained by Eran Dinur  in the september edition of 3D World magazine.

The filer used in the link given above is a "map" filter, and is difficult to use. Instead, use a "multiply" filter and use the value 0.8. The mixing of the materials should be around 44 to 46% towards the white foam material, which must be the first material.

This will make sense once you follow the link stew451 gave you.