Why bother doing these? This practice gives you insights into how to simplify masses, treat edges, what elements to keep and what not to keep and is a strong method for developing personal style. Better yet, you learn to use elements and masses for strong compositions. Before you even begin your final masterpiece, you have already done a plethora of problem solving and can paint with panache.
3-shade study using raw umber:
Paint a study using 3 shades using no white or black. Premix 3 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 help me to see the underlying design of a photo, and to simplify as well, I make a 2 and / or 3 shade notan using Photoshop’s posterizing tool:
Use your 3 shade study as a grisaille underpainting:
An option is to use your 3 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 3 shades each:
Following is another study based on the original 3-shade raw umber study, this time using a complementary pair of colours on a coloured ground.
I prepared a canvas board with a transparent ground made with transparent pyrrole orange which was glazed with dioxazine violet to dull.
Using the complementary pairs of quinacridone red and phthalo green plus titanium white, I mixed 2 sets of three shades (light but not white, mid-tone, darker but not black) of greys onto my palette. Each set of 3 shades had one redder and one greener pile (warmer and cooler). Having a warm and cool set of shades allows you to change your colours within a tonal mass without breaking your design. This is a great way to challenge your eyes to read shades of colour, which can take a lot of practice. Use your raw umber study to ensure that you are mixing the correct shades for your piles of grey colours.
The light accents and darks were added as final touches. 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 try 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, remembering the goal of maintaining the composition created in your study. You do this by keeping your mass values intact. As in all paintings, decide whether your painting is dominantly warm/cool, bright/dull and dark/light (low, mid or high key).