Students loved mixing the Primary Color Pastels as an experiment in Primary Color mixing. They chose one of the three Primary Colors which are Red, Blue, and Yellow. They drew their favorite shape and colored it in solidly using the same color pastel. The next primary color shape was added in the same way as the first, this time the two shapes needed to "overlap"" a tiny bit. I displayed the concept "overlap" by placing one hand over the other a little bit to give them a 3-D view. What resulted was one of the three Secondary colors ; Orange, Green, or Purple. They repeated using the next to pairs of Primary colors to create all the Secondary colors.
   Other skills I reviewed while working with pastel with first graders were:
How do you clean off a pastel when it gets dirty?
Scribble on the back of your paper, until it looks cleaner, repeat as needed. 
What do you do it the Pastel isn't working?
It's probably the paper around the pastel that needs to be turn down.
Pastels are very soft and get worn down quicker then crayons. 
The softness of the pastel is what makes it an easy tool to mix with.