Here's the second step of the portrait I am copying by the artist Ingres. This completes the first layer of underpainting. The whole face has been painted in a transparent layer of brown tones. Notice that just the shadow areas have been painted so far. The lighter areas will be painted next using much brighter paint. (Similar to his shirt collar). But the painting will stay in this monochrome tone for the next step. Also notice that the shadow areas of his face are not very dark. Compare the face to the jacket, see how much darker that is ? This is intentional, so the final layers of paint on the face can be built up to rich, colorful tones. This is the way Ingres would have created a portrait, there are many nice examples of his studies and unfinished paintings which are helpful tools to learn from.
