In the trunk of the tree, I scraped while the paint was still wet.
Scratching when the paint is wet creates a groove in the paper that will fill in with pigment to create a dark area. When the paint is semi-wet, the scratch marks will not fill in completely, leaving a lighter mark of the color that was scratched into.
I scratched into the foreground in areas where the paint was dry. You can remove all of the paint, right through to the white of the paper when the paint is dry.