Comment on demonic by Danny-Cruz

yes, that's it exactly.  The first impression is conveyed by the tonal scheme.  It provides the basic information from which one begins to make sense of what he sees.  They say 90% of the information we use to recognize a form is given in the silhouette.  The same idea applies to tonal scheme with regard to the entire composition.  

That it comes as an afterthought simply means that you're oriented to drawing, modeling, sculpting forms, primarily.  When you step back from the work or squint your eyes so that none of it reads clearly, such that all the detail is blurred and the only thing that reads clearly are the values, then you're orienting to the composition as a whole, as a painter.  This is also the level at which to deal with design.  This is what they call the postage stamp test - reduce the work to the size of a postage stamp or view it from a great distance.  If it falls apart, or doesn't read well, it fails.  The design should read strongly.  If it doesn't, or there isn't one, that's a big problem.  That is a pretty good indicator that the viewer must work too hard to get a foothold in the composition.  The comp should catch his interest, lure him in, then direct him here and there and keep him from leaving.  It's a seduction.

This is what I was getting at when I asked about digital.  You can work separately on building design skill by scanning drawings and playing with tonal schemes in PS.  I did that with some inks by (his inks over Mike Zeck's pencils of Colossal Boy) that I recreated. (see below)  Some people have a knack for it, the rest of us have to study and practice.  LOL.