I found the following images on Anne Kullaf's Blog Loosen Up. It is an excellent example of underpainting and the development of a painting.
first image shows an incomplete underpainting that resolves issues of
proportion, and begins to address value. This underpainting was done
using a tinted ground (gesso and some light red ochre) and burnt umber.
The values are beginning to be established by the use of washes to
block in medium and light values and an undiluted application will
block in the darker values.
in this slide the underpainting is blocking in the rest of the
background information. Underpaintings are meant to establish a firm
structure for your painting but do not need to be resolved and finished
in the same way that a finished painting is finished. Notice how loose
and gestural the figures and marks are.
slide illustrates the initial blocking in of color. Notice that we are
working with large simple blocks of color or shape, always work from
large simple shapes down to smaller more complex shapes. Resist the
urge to jump into details.
color is being introduced. Notice that the artist is starting to work
into smaller shapes and that some areas of the underpainting are
showing through. You do not have to always use a neutral color but you
can see here why it may be helpful because it is harmonious with the
palette being used. Imagine if the preliminary underpainting was
completed in a bright red or neon green and imagine the effect that
artist is continuing to refine the painting and developing a nice range
of values by mixing tints tones and shades of color. Edges are being
cleaned up and objects are brought to a higher level of clarity.
is the finalized painting. At this point the artist has provided "eye
candy" using fully saturated jewel like colors to add details like the
lettering in the sign for the restaurant.
is a detail of one of the figures in this painting. This is a wonderful
example that sometimes less is more. The face of this figure is merely
suggested through tonal variation as opposed to being meticulously
rendered. As an artist you will have to make decisions about how much
information to supply the viewer. This is an extremely useful tool to
guide the viewers eye around your work.