In this sketchbook photo I used the white of the paper for my whites and lightest values, then the 50% grey pen for the next value, and lastly the black Sharpie to indicate the shadow/darkest value.
I painted this using only my value sketch as my guide. This was an enjoyable exercise for me. I really had to think when choosing my colors that were back lit and in shadow. I'm not sure I did a good job of showing this, but when I converted this watercolor to black and white, below,