: Is cobalt blue a bad choice for text? From the introduction of The Best American Infographics(2014/Mariner): "Did you know, for example, that cobalt blue is a bad choice for text, or thin lines,

@Kaufman565

Actual cobalt blue (CoO.AlO3/PB28, as opposed to blue that merely looks like it, usually labeled something like Cobalt Blue Hue) has a very spiky spectrum consisting of widely separated lines at both the blue and red end of the scale. In broad masses, that's not a problem, but in fine lines it will cause scintillation as the eye attempts to focus both the blue and red at the same time. The effect is similar to the red and blue dots/dashes that Impressionist painters used to make their skies appear brighter than they could possibly have been painted with any single colour.

Cobalt Blue Hue, on the other hand, has a fairly blocky spectrum that occupies most of the shorter-wavelength (blue) end of the scale, tapering off towards the red end. In terms of being "just a shade of blue", it's difficult to tell the difference, but the optical properties aren't the same.

It's not so much that a particular shade of blue is a problem, but that a particular pigment will be. That should only ever come up in print, and then only if you are (for some inexplicable reason) using spot colour that happens to be based on a particular expensive and semi-transparent pigment (something you'd likely have to go out of your way to demand of the printer, who'd much rather mix the "same" colour from the Pantone recipe), or if your idea of "infographics" is something that applies to glassware or ceramics (where the high-temperature characteristics of the pigment make it ideal). Whether or not the information should have been included in the book is a question I will leave to the reader.

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