well, when i was learning to paint, i too struggled with how skin color can appear dark in shadow, but at the same time, pale. I suppose the answer has to do with how unsaturated and saturated colors appear in relation next to each other.
To learn this, I would take a professional painting into photoshop, and then drag my eyedroppper tool around the skin. As I did this I would have the color picker window open, and I would watch as the colors became grayer in some areas, and much more saturated in others (for instance, a cheek appears much more gray compared to an area that has a reflected shadow on it.
some good paintings to do this with would be something like this, [link] by Craig Mullins and this, [link] by Dice Tsutsumi
For practice, I used to color Disney coloring book pages in photoshop, using the skin colors from a painting like the two examples I showed you. I would try to do it on my own, but if I didn't know what sort of saturation to use, I would take it from the painting directly, and have an "a-hah!" moment. After doing this enough times, it started to make sense to me, and this is how I began to learned.
for example, heres one i did: [link], and I took the colors from this painting, [link].
Woohoo! A background. I think you should define the flowers in her hair more. And there is a bit of unexplained highlight on the left hand side of the brunette's face. There are like, two stripes of blue there under her eye beside her nose. It's coming along great though.