I showed them a few examples of warm and cool colored backgrounds I had made and asked that they guess what I used and my technique for using it. The kiddos were spot on! They could tell that I had used oil pastels and layered the colors and also went over them with either my finger or a tissue to blend the colors.
I also showed them a finished example with a silhouette on it. I asked them to describe what my silhouette was and asked them how they could tell (it was an owl perched on a tree branch). I wanted them to think about this because I wanted them to realize what details were needed in order to identify a silhouette and how our eyes can still use those clues to tell the difference between one subject (in my case the owl) and another (the tree branch) without relying on drawn lines or too much detail.
Once they seemed to grasp the concept, I had them start their backgrounds. I asked them to decide if they wanted a morning or night sky before they started, and this would determine if they worked with warm or cool colors. They covered an entire sheet of construction paper with their chosen color scheme and blended the pastels (some better than others).
I gave them black construction paper and white colored pencils to draw out their silhouette. I had to remind them that the pencil side would get glued down and that they didn't need to add any details like eyes, clothing, etc. They cut and glued their silhouette onto their background and we were finished within the hour!
I love the variety of subject matter they came up with. I go back and forth on requiring certain subject matter, in some lessons, I think it is important to narrow it down for the students. But in this case, I think the freedom suited them well. All of their work is pictured below with a few close ups for details. Enjoy!