Verdaccio is an Italian name for the mixture of black, white, and yellow pigments resulting in a grayish or yellowish (depending on the proportion) soft greenish brown.
Verdaccio became an integral part of fresco painting, in which this color is used for defining tonal values, forming a complete monochromatic underpainting. Often architectural details in frescoes are left in verdaccio without any additional color layers; a notable example is the ceiling of the Sistine Chapel, where verdaccio underpainting can be clearly seen, left as is on all architectural details of the composition.
In oil painting, a similar technique is used by Flemish painters, referred to as the "dead layer," is applied over the traditional bone colour (1 part raw umber to 1 part yellow ochre) priming to refine the values and remove the warm tone of the primer. The resulting "white to olive green to black" underpainting is "like being illuminated by moonlight." It does not affect the tonality of the final painting.
This method was something I learned in undergraduate school. It was hard at first because we had to follow very stricked rules and once the underpainting was dried, we applied transparent colors and then opaque colors with highlights being the last application. My first verdaccio painting was a still life, then a portrait and now, I am using it in landscape but not in the same stricked sense. It really helps to establish values. It is easier than using all those color chromes and intensities that you have to use when painting in a direct manner.
You can see how I altered some of the values with the opaque color as I went along for a better effect. I am not done yet! I will post that one when the painting is completed.