Comic book pre-press history lesson time! Who remembers what a Color key is…?  A Color key is an overlay proof created from the film separations that places each ink color on a separate clear acetate sheet then assembles them together over white paper.  Color Key overlay proofs to check registration, trapping, and color breaks, identify density problems.  They don’t really use color keys in the prod. of comic books anymore.  Everything is proofed digitally.  Here’s how it worked:  A colorist would color the cover, then send the files to a pre-press house to have color keys and film made.  The pre-press guys would then manually “strip” together each film, using a loupe to ensure accuracy. Then, the color key would get sent to whichever editor was handling the book at Marvel / DC.  They would make notes, corrections, etc., on the color keys, and send them back to us to fix.  The whole process took about 2-4 days.  Nowadays, all that happens in minutes with e-mail and jpgs.  I came across an old color key for a Spirits of Vengeance cover that I colored back in May of 1993.  Over time, some of the ink has started sticking to the other sheets of acetate, but you get the basic idea.