I don't know that it completely resembles marbling. Did you take a comb and rake it around to spread the paint? Did you spread out the shaving cream so it covers an area as large as your paper? My students enjoyed the process, as well as the chalk on water and the traditional paper marbling. I had 3 stations set up and had them try each method.
OK guys! I tried it twice. I followed Leah's directions. I did it with watered down tempera and then with watered down watercolor. There is no question about it. If it looks like marbling, I am fooled. Am I a ATF? (art teacher failure). What am I doing wrong?