The white pastel can draw white on top of the charcoal, so now I can make white marks, which can also be smudged and mixed. It’s giving me a range of values I couldn’t get before. Throwing white highlights onto a dark drawing is a way of directing attention and makes it more interesting.
Black and white, on a drawing, are the extreme values. If I try to apply this idea to writing, it should also be a juxtaposition of opposite extremes.