While looking at different web sites on oil pastels, I found one (here) that discussed this technique, using water based oil pastels, called Sgraffito, which is Italian for scratch away. Sgraffito can also be used with paints and pottery.
Color blocks of color (in any shape) with "light" colors of water based oil pastels on watercolor paper. I dipped the pastels in water, which intensifies the colors.
Completely cover the surface with black water based oil pastel.Use tools of various width to scratch a design through the black layer.
Here is my doodles sgraffito sample. I added a rhinestone to the center of the larger flower.



