Swatch where you are going!

I like to keep a record of the colours I mix when painting by making little swatches and writing notes as to which hues I used and how much. This practice has been very helpful lately when I'm trying to match flesh tones on paintings that I revisit. Lately I've been putting the swatches right on the same panel as the study and it does something kind of interesting to how you read the image; your brain starts to try a piece together which swatches go where. Neat! I wonder if anyone else has done this as a "final" piece?
Do other painters keep a record of their swatches?

After Jenny Saville