Thanks all for the comments.

Rony, yes one could do each primary color first and combine, but color definition and density also involve whites and greys, so the additive effect, by the time one plays with it, would be quite a bit more dense for some parts of typical objects and would get complicated to correct. Also, the eyes are really good at interpretting density and hue changes over a spatial area because of the way we were created to see and interpret. It is quite a process that occurs and sketching the colors at one sitting take advantage of our quick abilities to interpret changes. To me, its more a question of getting the interpretation on the paper that is not easy (I see what want to do but can't quite get there), but thank goodness for blending stumps and using them to apply as well smooth color. I am getting quicker but the learning process will probably take a life time. It sure is fun learning!!

I prefer, also, to stay away from using computer techniques as a substitute for my steps of creating the scene. That is just me, of course, since many prefer the opposite. I limit the computer work to scanning my failings or my strengths.