Despite using a restricted number of paints, the chromatic scale is wide. Including dry pigments as well as paint or different textures from different matters, which bounce and capture the external light each in theyr own way.
Black for fresco is mainly reserved for the sky. It is used as a load in diluted paint, allowing the cotton to be tinted in mass. Light modulation is made by addition of dry pigments: Black for fresco for darkening, Titanium white for lightening. Thus, all the mass of the sky is made with a unique paint reference.
Cemented portions are made with cement tinted in mass (mix of cement and Ivory Black pigments), then tinted again with an underlayer of wood stain before the colouring process. Some tinctured portions are left exposed, allowing to get a very dark warm grey-brown.
Warm tones with greenish grey dominate in the top half of the painting, especially on the sky and the illuminated part of the wall. Warm tones still remain in the bottom part of the painting, but are conterbalanced by cold touches of grey-blue, more present in the shaded area of the wall and especially on the ground.
Lighted parts are made with a mix of plaster, Titanium white, and glue. Then they are progressively graded with light glazes of grey.
These multiple combinations of pigments/paints/matters are giving in the end a very wide chromatic range.
Black is not unique. Several different blacks have been used and combined with different matters: black for fresco as a paint load in the cotton, Ivory black mixed with artificial water or used as a load in paint on mineral matters. These blacks are giving the maximal opposition against the mix of plaster, glue and Titanium white.
Intermediary greys are given either by the matters themselves, either by different paints, either by modulation with loads of pigments, either by the external light which catches the volumes and cast real shadows in the hollows.
That's how shadows are built both by pigments and by the casting of real shadows.