I couldn't survive teaching elementary art without cray-pas. They're the best.
I first saw this idea when I was at an international school in northern Thailand for an Arts in the PYP workshop. I remember it well because it was my first time away from Lucia, who was only 8 months old at the time. :)
When I tried it with my kiddos, it was part of a larger color unit and quite simple once the silhouettes were all traced. After lots of front loading about warm and cool colors and what kinds of emotions they can express, students picked out either warm or cool cray pas to fill in their silhouette. Of course there was also front loading on oil pastel skills-- blending, layering, etc. I just love them!