Why not green?
What are these comments? "Nice word choice" "No changes here" "This is perfect" "(smiley face)"
EDIT: Also used for printing instructions, to make sure that things come out in the right order, for instance.
You put the page with the correct word underneath the copy, on a light table (a real one). With an exacto knife, carefully cut out the word, cutting through both sheets. Now the correct word fits exactly in the hole. Hold it together with a piece of white tape (standard supply in all these shops) on the back. The mend is invisible.
Ben likes to actually run controlled experiments and measure things like that, instead of just speculating! ;)
See the email quoted here: