Yes, it's painted from scratch (with photo references of course) I've tried making tutorials before, but they're not really "my thing". They usually end up 'not so good' and/or unfinished.. So instead I would like to redirect you to my Tutorials ol. folder on my Favourites page.
Yes, I use multiple layers, some with color and some just with hue (the hue over the color layers of course) I also use some adjustment layers like curves and hue/saturation/brightness and mask in/out parts as i see fit.. i feel this gives me a bit more control and it's fast and easy to experiment with ^^
A nice tric, just to give the skin a bit more life, is to play around with the hue jitter on your brush to create a bit of variation ^^ just like a couple of few percent.
For the background i used only the color gradient from the WIP image (the one with colors) and then played around a bit with some adjustment layers with heavily blurred masks. *If you make the adjustment layers into clipping masks for the color layer, you can play around with curves, contrast, saturation and so on without affecting the luminance of the other layers below.
... other than that, you just need to practise, look at photos, experiment, and have quite alot of luck when choosing and mixing colors.
Remember that the midtones for skin is often quite saturated (compared to lighter/darker tones) Also, paint in layers to achieve depth, though i find this kind of coloring to be easier if you do it straight away, and not over a black & white image, since you often end up in the dilemma of not wanting to change your brightness/values.
Hope this wasn't too long and/or unrelevant. Thanks for commenting, hope this helps, and good luck!