Picasso did all his paintings in 5 colors or less. He said it was easy to
do a painting in more but a true artist only needed 5 colors. Try having them
do a cubist still life using musical instuments using warm and cool colors
and limit their palettes. Remember cubist still lifes showed the objects from
different sides, so maybe a charcoal drawing from three different views of
the same still life could be combined as a preliminary comp. The combined
views together could be made into a cubist painting. Also the same shapes
seems to be repeated in the painting for unity.