/ic/ - Artwork/Critique

What is the "secret" for picking the right saturation on the darker areas?
I have some theoretical knowledge about it, mostly from either books, or tutorials, i'm not quite sure at this point.
I know that there is a transition between light and shadow, an area were the saturation (and darkness, i think) is higher. I also read somewhere that the shadows are supposed to be less saturated that the lighter areas (i'm not sure about this), yet, very often i see extremely good paints, where the darker areas seem to be sometimes more and sometimes less saturated, even to the point of being almost grey... can someone tell?