During my two week practicum we made shadow trees on a sunset background.  My students painted the background with water colours starting with yellow, then orange, then pink, then purple and ended with blue.  Once the paintings had dried we created trees with black pastels.  The students enjoyed this very much and it made a cool image.  In the future it might be cool to do a four piece image.  One image with a blue background, one with a pink background, one with an orange background, and one with a green background.