How local color shifts as it moves from shadow into light

A little-understood phenomenon, that I call a hue arc, is on my mind this morning. I'm completing my first paint mixing tutorial, on reds. 

I've observed this hue arc for almost 20 years. At first I questioned my perceptions and doubted myself. But I saw them so frequently, and could illustrate them with a bit of Munsell magic, that eventually I named them hue arcs or spirals. 

How important are they? That would be up to each of us, but I love the way I can paint the color of light by seeking them out and articulating them with my paint mixes. 

 The above image shows the dark red values at the warmest of the reds, 10R. As the values rise (get lighter) the hue shifts to 7.5R, then 5R. The highlight shifts again, to 2.5R. In other words the hues are moving toward red-purple. I find this fascinating. 

This image exhibits a similar arc, but starting with 7.5R in the shadows and shifting to 10RP (red-purple) in the highlights. Both photos exhibit the same sized hue arc. The only difference is the starting point. They each travel through 4 hues. It's possible to study any object/subject and quantify how the hue arcs exhibit.

Putting in a little time, making some notes, gives a painter a way to approach any object every time. Since I have two examples of this that describe the way red lipstick shifts then I can begin with any particular hue I want and shift it in a similar manner. 

The same thing is true of painting anything else. Find the local, analyze the hue shift and plot the color shifts as you want them to be. For instance, in the painting below, the values in the lips are not as dark as they are in the two paintings above. What I did is keep the same hue spiral, in this case from 7.5R to 2.5R, but shifted each value up one. Could have shifted it all down as well. 

This is exactly what I've done with the painting below, which is not finished. I'll be recording my painting of these lips this afternoon and will share it here.