The designer's invisible pencil mark, a pale blue that cameras can't see.
Non-Photo Blue is a pastel blue, close to cyan.
It can be described as pastel, soft, light and rather unsaturated.
Set in black, because black reads best on it.
Non-photo blue is a common tool in the graphic design and print industry, being a particular shade of blue that cannot be detected by graphic arts camera film. This allows layout editors to write notes to the print operator on the print flat which will not show in the final form. It also allows artists to lay down sketch lines without the need to erase after inking.
Read on WikipediaBlue is one of the three primary colours in the RGB (additive) colour model, as well as in the RYB colour model. It lies between violet and cyan on the spectrum of visible light. The term blue generally describes colours perceived by humans observing light with a dominant wavelength between approximately 450 and 495 nanometres. The clear daytime sky and the deep sea appear blue because of an optical effect known as Rayleigh scattering. An optical effect called the Tyndall effect explains blue eyes. Distant objects appear more blue because of another optical effect called aerial perspective.
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