Hm... Do take my answer with a grain of salt since, like so many other artists, I'm still learning. >.> ;; Then again, aren't most artists always learning? That, and different artists tend to have different answers for just about any art questions. @_@Sorry to say, but there is not necessarily one "right" way. A darker shading colour could either get more or less saturated depending on many, many factors (Light source, how realistic you would the drawing to be, and base colour scheme to name a few of them). Though, as far as I know, you're not breaking any rules (spoken or not) that I know of, so there's that.But even then... the rules can be broken as long as you know what you're aiming for in your drawing.