While Underpainting, there was no color to think about. The painter could devote all of their time to working on the tones of the painting. An underpainting really can be classified as any paint that has dried and is again painted upon. Your underpainting is where you build your foundation. Your main areas of light and dark are planned out in your underpainting.
Some people paint an underpainting in one or two colors and some use a full palette.
Personally, I like to make an underpainting in one or two colors. I will establish the main part of my painting and worry about color later, when I can devote my full attention to it. Think if you had to type a letter, do a math calculation, and cook a meal, all at the same time. Painting provides different problems, but they are definitely problems. If your colors are wrong, you won't be happy.