Addendum: Sorry, I got my sources mixed up! I saw this exercise, using a DPW photo, from Jo's downloadable Visual Guide to Painting (which is on sale this weekend); ten lessons with links to her videos on YouTube.
You can see Jo's work and lessons on YouTube and on her website.
I came across this old Daily Paintworks challenge on Jo MacKenzie's website. I have long admired the clarity of the values in her paintings, so I decided to give this challenge, which she credits with helping her see values more strongly, a try.
The challenge was to change the color and paint the value. I didn't change the colors (much) because I couldn't decide which colors to use! But I think I will do it again, the way it was meant to be done!
Melo, interrupting my photo-taking.