Like pre-school but without the children

Back to the drawing board after a long time of not working on this project.
I think color highlights will be a good way to make parts of this piece stand out. I start with the water soluble wax Caran D'ache crayons but they are too stiff and large for the small areas I want to color.
The Staedtler Karat Aquarell pencils are perfect for this project. The point can be sharpened just as a traditional pencil, but the difference between normal color pencils and these comes with the addition of water. Here I am using droplets of water that are on the edge of my cup. It is just enough liquid to make the colors soluble.

A wet, fine tip paintbrush helps blend the colors but in most places I leave them rough looking and not blended. The gesso background is keeping the results from looking like watercolors- which is normally the case with these pencils. I like the look so far...
I just don't know how much to fill in. In the beginning I thought a few areas of color here and there, now I'm not sure how much more to do. Any suggestions?