Here are some examples of Nelsons Shanks "duo tones", or limited palette paintings. Actually, the term "duo tone" is a misnomer, as there are more that two tones. These studies are probably more correctly termed "closed grisaille", as the initial empty light masses of the brown grisaille paintings are filled or "closed" with color representing the lights. The shadows remain as the grisaille mixture. At Studio Incamminati we generally use burnt sienna, ultramarine blue and a little white (if necessary) for the grisaille. This process can be used for completed studies, or can be part of a procedure for developing a painting (see Kerry Dunn previous post for an explanation of closed grisaille).
I included this example of the beginning stage of one of Nelsons' paintings, so you could see how the "duo tone" studies relate to the opening stages of a painting.