This is a tool called a Value Comp (found at art stores online) that can be used to find the "lightest lights" and the "darkest darks" plus all the grays in between in order to plan the painting. If you follow the green lines from the left squares to the arrowheads, you'll see the grayscale values for various parts of the reference image. This is one method that helps you understand what value belongs where in order to give a subject volume and mass. It also helps to think in terms of areas of blacks, whites and grays, without getting distracted by the all the color and details.
Rarely is there a pure pure white in a subject, for example. The blaze on Miss Wickers is very close to white, but I would paint it using Titanium White and probably a touch of Payne's Gray or Burnt Umber/Burnt Sienna. If anything, I'm going to want to "push back" (decrease the value of and attention to) the white blaze and bring the viewer's focus to a highly contrasting area around her eye. Doing this makes me in charge of how the painting is viewed.