Understanding Colors

[bcripps]

   
I've been Gimping along for a few years now, but never got my head around the Colors drop down selections. Experimenting has just added to my confusion... all those choices, separately or combined, and I never seem to obtain the desired results, particularly with skin tones.
Here's an (extreme), example of a head swap with the complication of skin tone mismatch. The face is light and reddish in complexion. While the shoulders are dark and quite brown. I tried Curves but it would appear I need to adjust the coloring first so they more closely match. How do I get rid of the red? Add brown? Or vice-versa...
I would appreciate any suggestions that will help me better understand Colors.
Much thanks...
Bry