Hi Bill,
I’m using Free Select Tool:
How to select skin areas is described here in great article about skin retouching by @patdavid :
I should have been more precise. The issue is selecting around hair. Maybe your selection just skirted near it.
Yes, as in the picture below from the tutorial. If there are some hairs in the selected area, wavelet decompose makes sure that they are preserved when you work precisely.
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Thank you, Boris.
Gotcha. Thank you.
Bill
An ancient rule for converting a color photo to black and white is to mix the three color channels in a certain way to get the desired result. This method can also be used to influence the color contrasts of color photos.
GIMP offers a lot of possibilities to do this. I’d like to highlight two tools that are very useful in this regard, mono mixer and color balance tool.
First mono mixer in combination with luminance blend mode. This blend mode ensures that only the brightness and contrast of the colors are affected and not the saturation or hue. LCh Lightness blend mode can also be used. However, this also increases the vibrancy of the colors a bit.
Ok, here is our initial image:
The setting is quite simple: We duplicate the layer, assign it the luminance blend mode, select this layer and open Mono Mixer from the color menu in the upper bar (Colours - Components - Mono Mixer) :
Now a few examples of what can be done:
darken blue areas and brighten red ones:
brighten green areas:
darken red areas:
As you can see, a lot of combinations can be made to influence the color contrasts. I highly recommend experimenting with them!
Color Balance tool is also very well suited for this purpose. Its advantage is that one can apply color contrast separately to three different brightness ranges - shadows, midtones and highlights. This allows countless possibilities of color contrast influence!
That’s why I will concentrate on this one picture and everything else you can try for yourself. 
Setting is the same as before, instead of Mono Mixer we now use Color Balance tool (Colours - Color Balance):
These are the changes I’ve made (Shadows, Midtones and Highlights):
And this is the result:
And after I have increased the saturation a little bit:
This technique is also ideal for processing portraits! Just give it a try. 
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Thanks for a great writeup. I usually tweak the LH (lightness according to hue) setting in RawTherapee’s LAB controls. RT’s RGB controls do something similar. I’m not sure of the practical difference between the two, TBH (in RT, that is).
Question on the first part of your post where you are using the mono-mixer + luminance layer blend mode: is there a way to use this technique if the 3 dominant colors of your image are cyan, yellow and magenta? In other words, not the primary colors from the RGB color wheel?
Yes, basically blue channel slider influences cyan, red channel slider influences magenta and green channel slider influences yellow whereby also the combination between the RGB channels is important. To see exactly what is happening, I recommend downloading a color wheel and playing with different color channel combinations 