Many artists believe anyone wanting to exercise or perfect their drawing skills should sketch in black and white and temporarily suspend the use of color. Likewise, foregoing the distraction of working with more complex things such as paint may be necessary. Charcoal and graphite work very well in this pursuit, but a lot of my own drawings appearing on this blog will just be done with a number 2 pencil. I can tell you that working with just a pencil forces you to balance your values right out of the gate. One way to adjust value with a pencil has to do with the amount of pressure you apply when drawing. The same adjustments in pressure and resulting value shifts can be made with soft pastels, which I often work with, too. It is critical when working with pastels to know when to apply a "light hand", and when a "heavier hand" is needed. Here are some recent faces I have rendered in soft pastel.
approx. 9" by 12" on pastel paper
"Mr. Blue" 18" by 24" on pastel paper
approx. 9" by 12" on pastel paper
Pastel value study, 18" by 24" on pastel paper