Roland,

If color sketching is too complex:
Would you be interested in separating the colors from your monitor?
The procedure is simple at the sketch, but more complex at the desk.
I guess that your monitor can have its colors adjusted in a way that only one of the RGB's comes through. That way, you can concentrate on one color at the time.
You could create 4 sketches in B/W: one B/W, a green , a blue and a red one. Then you can color each sketch and merge together, both coloring and merging with the computer.