There are certain tricks or techniques an artist can use to enhance the illusion of 3d form in their work. Picking any number of these tools from the mental drawer can drastically improve a painting, and they prove especially helpful in realism when an artist develops the intuition to deviate from the reference where necessary, in order to make a more successful painting (as opposed to a more successful reproduction). One such tool is knowing where and how to reduce the chroma (or intensity) of an object’s local color as it recedes into shadow. In the words of classically-trained figurative painter Shane Wolf:
It’s crucial that the lighter value mixture be slightly higher chroma than the next darker mixture as this is how form works in nature. “Lighter brighter; darker grayer” is an old academic saying that explains that if an object is in light, it’s brighter; as it turns away from light into halftone, it gets darker and grayer.
(Wolf, Shane. “Alla Prima, Three Hour Sketch.” The Artist’s Magazine. November 2012: 38-39)
We all know that shadows make colors darker, but what we often forget–myself included–is that to complete the illusion in a painting, it may also be necessary to dial back the brightness/chroma/intensity of local colors to avoid shadows that compete for depth-space with foreground surfaces or objects, to the detriment of the overall image.