Thanks, John"

Hi, John,
There are kind of two different principles at work here, and those principles sound similar, so they can be a little confusing.

1. One is that the darkest values on the lit side of a given object are almost always lighter than the lightest values of the same object in shadow. This is assuming that the object is of a fairly constant local color and a fairly matte surface, such as a skull, fabric, or skin. It would not apply to something patterned, glossy, or highly reflective. It's also assuming we're talking about sunlight or any strong light source with normal surfaces bouncing the light back into the shadow. Given those constraints, this one is nearly always true. You seem to be adhering to the principle in your picture.
2. The other principle is that in a black object lit by direct sunlight can often be lighter than a white object in shadow. You have also got that working in your picture, as the light side of the black cloak (left swatch) is a little lighter than the shadow side of the skull (right swatch). In my observation, this one only holds true under ideal conditions. Outdoors, you need to have a cloudless sky and not too much reflected light coming into the shadow side.

So as you guessed, the first rule applies to one object at a time and the second rule applies to the whole picture. These principles come up because students tend to underestimate the depth of shadows. They also tend to introduce too much tonal variation within the lights and too much tonal variation in the shadows. This happens because our visual brains use context cues to override the luminance information that our retinas actually receive.

You also mentioned a concern about maintaining the appearance of the local color of the skull as it moves from light into shadow. A white object can move through a wild range of colors as it absorbs different influences around it. I'm guessing that reflected light from the yellow tassel would spread a vertical glowing band of warm light—thought too light in value—to the area of the shadowed skull just to the left of the tassel.

One more thing: Can you get that student on a better meal plan?
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Previously: Black is Light, White is Dark
More in my book: Color and Light: A Guide for the Realist Painter