Painting a small color study is so essential to starting a bigger painting. I really have embraced this process. The only times that I would skip painting a color study or a composition study when I am so sure of what needs to be done and because I need all the days that I have with the model.
Another good reason for small study is that it gives me time to plan for an appropriate painting surface. I usually have a few different canvases or panels ready to go, but sometimes I may need a few days to prepare one.