re: #250 ObserverArt

Cyan markers do not show up in graphic arts film used to compose type and halftone photos and then to burn printing plates. They may have used them from being around the paste-up artists that also used cyan for drawing guidelines etc. on the paste-up boards.

Ahhh, the old days.

The cameras used to image the (B&W) pages back then were not sensitive to cyan, especially one special shade used for markup pens.