This technique, called sgraffito, allows you to partially bring out a color by scraping the one on top of it. Use the tip of a knife or a utility knife to create the patterns you want. Have fun!
What you need to know
Sgraffiti are especially well-suited to oil pastels, preferably applied in thick layers.
This method is also useful with dry pastels, provided you fix the first color before applying the second one.