Made me curious and had to play... fwiw on Win10 if there is no image defined in "Non captioned window edge"
Dave, yes that controls the color. But it can also control many other areas especially some border colors.
As far as the non captioned window edge image. Not sure what WB you are checking or using that it doesn't have an image. Most everyone I know uses an image in that location. In win 7 it does nothing so maybe that is why some skins don't have it in there. I have always used an image in there along with many others.
IN win 10 the image will control that menu. It does help to in most cases match the skin better. One reason is most of the time you would want the ButtonDkShadow color dark and it doesn't match many skins.
Try one of my skins, Lightstars, Don's, who ever that use the non captioned image. Again which most of us all use. Check the menu then remove the image and apply. Now you will see the color.
The reason I didn't mention the color location in the thread is because as I said most have an image in there. So changing the color you will not see any change. As far as the skin you say doesn't have that image I don't know what it is. I see only a couple made mt Stardock Design. So what skin are you using?