I love that!
I tried another PKR exercise based on that idea today.
First, we painted a value study of a still life using burnt umber and white.
Then we tried to match the values in color while doing the same set up.
We mixed the color we thought was the right value, then dabbed it on the value study to "check it".
It makes it pretty clear how far off you can be when translating color into values!
This is a lot harder than it looks, but I recommend trying it.
Note to self:
If I do this exercise in a workshop, I will use one object in strong light.
We all spent way too much time drawing this complicated setup,
so it was hard to stay focussed on the exercise!