I think you are mixing together two different approaches here. If you are doing an opaque monochromatic underpainting you are setting up your painting to be glazed (no white). Therefore your underpainting needs to be lighter than the desired result since all glazes darken. I am not a proponent of this approach, as I have often noted.
Scumbling, as I practice it, is based on applying translucent layers of paint over previously built up color in order to unify areas and to give them a translucent glow. I have a demo on my web site and you can see the effect of this demonstrated there. (second row from bottom, right side image)
http://www.fineartportrait.com/workshop_demo.html
The painting in my demo was not completed due to the time restraints of leading a workshop but in real life I would continue to rescumble and define until I achieved the desired degree of finish I was interested in.
Obviously if you attended one of my workshops the whole procedure would make a lot more sense since I explain things stroke by stroke as I go along.
It wouldn't make any sense to scumble one color over light and shadow since, I assume, you wouldn't want to unify and fuse both areas and therefore flatten the form.
Also the whites of her eyes are scaring me. Way too light!