One thing about oil paints is: don't work the colors too much or they will become muddy. Build up thin layers of color. Let each layer dry before putting on the next. Never put a dark color under a lighter color unless you want that color to show through the next layer. Use varying tones of your primary color for shading and highlighting. In other words, (for skin, use darker tones of your main skin color for shading, and lighter tones of your main skin color for highlighting.)
Shading: gradually make the skin color darker until you get to the darkest area. The opposite for highlighting. If you want a certain area to really stand out, you can put a highlight beside a darkly shaded area.
btw: I like the girl picture the way it is. I would try and keep all of the girls. Just fix them up. And I kind of like the darkness of your goulish picture also. Keep working on it, until you get it the way you like it. Good luck.
Art gives me a life of extreme challenge, frustration, accomplishment and contentment. Nothing less will do!