Last week, we did an exercise on simple value/tonal painting. Our subject was a raw egg and one or both of the eggshells on a white plate. We lit our our egg and shell and moved the pieces around to find an interesting composition. Using three values (light, medium and dark), we looked to have different quantities of our dark, middle and light tones to create interest. For example, in my monochrome example, there is a small amount of lightest value, a middle amount of dark value and the largest amount of medium value. As long as the quantities of these three basic values is different, the composition will be interesting. In contrast, an equal distribution of light, medium and dark masses in a composition will be uninteresting.
This is a figure taken from "The Simple Secret to Better Paintings" by Greg Albert (link) illustrating that given unequal division of the tones in a composition, the smallest area becomes the center of interest (regardless of its value)
Our exercise used any single color. Some made a grey using transparent red oxide and ultramarine blue, or pthalo green and alizarin crimson, burnt umber or the fabulous color asphaltum by Gamblin (link) in my example above.