I absolutely love these!!!
I taught students how to draw lizards. Then students used soft pastels to color. I encouraged students to use analogous colors (colors next to each other on color wheel) and blend them together. Finally, we used black glue to outline everything again. This is the hardest part. Make sure students are actually touching the glue tip to the paper or it becomes a mess. (I mixed Elmer's glue with black tempera paint to make this)
The basic idea for this lesson came from http://deepspacesparkle.blogspot.com/. It is one of my favorite places to get ideas.