Again, I am not going to properly describe the technique, but this is how I saw it done:
It was made with layers and layers of paint which is why it weighs so much. At first, it looked like just colors haphazardly and sloppily (I wonder if my son will appreciate my professional description) being applied. But then he started to carve out (for lack of a better word) certain parts revealing the colors that were deliberately chosen backwards to forwards to show through. Then he painted more on top of the rest of the picture. O.K. that's all I'm going to say because I'm not helping, but look at the end result!