In the Simplified Shadow Mass exercise you practice visualizing the darks and shadows grouped into as few shapes as possible and paint them in one dark color. Then you can vary the colors for the rest of the painting. I tried that in the two top studies.
In the bottom two I allowed myself to use two different colors for the shadows instead of just one. I like the last one best (bottom left) and the first one second best (top left). I hate the muddy second one (top right).
It was fun experimenting with massing shadows and playing with composition by sticking to two objects and changing only one at a time.