On the middle of a lesson I have understood that something not that with a color:) Whether somebody can give advice or the reference to a lesson about mixing of color?
From what i see, your biggest error seems that your skin is all the same shade, you just move the light/dark slider, while a real skin have a lot of different undertone color, yellow, blue, red etc. It's really important to have a good palette.
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Here's an example from the internet
And i will be glad to hear about another error)
http://stuff.veekun.com/pkcolor/