Why bother doing these? This way of mastering composition by painting three-shade studies gives you freedom. This practice gives you insights into how to simplify masses, treat edges, and choose elements worth keeping. In addition, it is a great method for developing personal style. Better yet, you learn to use elements and masses for stronger compositions. Lastly, you have already done a plethora of problem solving before beginning your masterpiece,  which frees you to paint with panache.

Three-shade study using raw umber:

Paint a study using three shades using no white or black.

First, premix three middle shades as shown above, noting that the lightest shade is not white and the darkest shade is not black. Do a quick pencil sketch on paper or canvas paper. Then block in where you think your shades should go. Keep it simple and avoid getting icky picky.

To simplify a photo,  I make a two and / or three-shade notan using Photoshop’s posterizing tool:

Use your three-shade study as a grisaille underpainting:

An option is to use your three-shade study as an underpainting on which to glaze or paint opaquely, matching the shades of the underpainting, plus adding the accents of white and black. Below is a partly completed study showing the addition of a global glaze of nickel azo yellow and opaque violets and blues. See how the yellow glaze creates a dull green colour on the raw umber. If you decide to use your underpainting as a grisaille (meaning shades of grey in French), paint lighter values if you plan to glaze as glazes will darken the values.

Use sets of colours, premixed into three shades each:

Following is another study based on the original three-shade raw umber study, this time using a complementary pigments on a coloured ground.

Here are the steps:

I prepared a canvas board with a transparent ground of  pyrrole orange. To dull the orange, I glazed it with dioxazine purple.

Using the quinacridone red and phthalo green plus titanium white, I mixed two sets of three shades onto my palette.  I made each set of three shades to have one redder and one greener pile (warmer and cooler). Why? Having a warm and cool set of shades allows you to change your colours within a tonal mass. Plus it keeps the tonal masses intact. Learning to read shades of colour takes a lot of practice. Compare against your raw umber study to ensure that you are mixing the correct shades for your colour piles. Like the raw umber study, the shades did not include white or black.

The light accents and darks were added as final accents.  Whites: titanium white plus a tiny amount of nickel azo yellow, and also very lightly glazed with nickel ago yellow in some places. Nickel azo yellow is a very transparent and bright acrylic paint. 

As suggested by David Langevin, I use fluid acrylics when painting with transparent and heavy body for opaques. Blacks: quinacridone red and phthalo green mix were glazed where needed. Carbon black works as well. Mixing your blacks gives you the choice of warmer and cooler. Alternatively, you can add colours to your carbon black. Acrylics dry darker, so I like to glaze in layers rather than adding too much at the get go. 

Add more colour:

When painting your masterpiece, you can further add colours to your palette. Keep in mind the goal of maintaining the composition created in your study. As shown above, you do this by keeping your mass values intact. As in all paintings, decide whether your painting is dominantly warm or cool, bright or dull and dark, and mid-tone or low or high key. 

Mastering composition takes a willingness to do the work. It is a fun process. Enjoy!