Thumbnails and value decisions

Today while sitting in the house looking out at the rain coming down I thought how nice it would be to be out painting en Plein Air on a sunny day. As I was looking out the kitchen window I noticed the contrast between outside and inside the house. I grabbed my 50% grey Prismacolor Pen and a black Sharpie to sketch the contrast in value's.

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,

it seems pretty true to my original value thumbnail.