Well, I am working away at my Carrie Fisher tribute and wondering whether I can do it to my satisfaction or not. I still find tone transitions hard in digital formats or in fact, actual physical paintings too. In pencil work, the task is much easier because all you need is to smear the tone out from the darker regions with a blending stump. If you wish to add some back lighted reflections you just erase some tone away with a kneaded eraser.
With paintings, you have to gradually adjust the darker tone to the lighter tone through a combination of correct color half tone mixtures and on canvas blending to smoothly take a color through the tones of shading. With digital painting in Gimp 2.8 the idea is somewhat the same except that on canvas blending is not really available as far as I can tell. With the smudge tool you can mix colors; however, there is one difference from physical painting. With physical painting, you can vary the amount of paint on the canvas, which helps the blending. In Gimp, on the other hand, you can go over a section of the painting as many times as you want, but the depth of color will always be the same. In addition, when you start blending the colors together, you start thinning them out. In other words, they become transparent. The more you blend, the thinner the mixed color. While I have found times when this is useful, blending shadow tones to light tones is not one of them.
For now, I have to decide if I am okay with the tones I have achieved on Carrie Fisher. At a distance, they look fine but closer up they do not look as good as the tiger in my last painting. Time will tell.