Colour

Explanation

The illustration on this page is an enlarged section of a book illustration. The book illustration is typical result of the 4-colour (CMYK) printing process. The enlarged section demonstrates how the cyan, magenta and yellow inks combine. (This particular section doesn’t happen to need the black ink.)

In one region a more-or-less pure red is produced by a combination of magenta and yellow inks. In another region, a bluish cyan is produced by combining some magenta with the cyan. In the third region, purple colour is produced by combining cyan with a larger quantity of magenta.

A slight misalignment during the original printing process also produces some revealing combinations. Wherever the cyan and magenta dots overlap, the result is blue. Where red and cyan regions overlap, the combination of cyan, magenta and yellow produces a dark brown that may appear black on this computer monitor.

Technical notes

• The process of scanning the book illustration and displaying it in a browser will, inevitably, have introduced inaccuracies in colour reproduction. The main points remain valid in spite of these inaccuracies— the inaccuracies on this page are insignificant compared with the inaccuracies in many books on colour!
• There’s a nice little irony here—the very illustration that demonstrates how red and cyan make dark brown is asserting that red and cyan make purple!¹ Yes, they call cyan blue, and yes, the red is made from yellow and magenta, but that doesn’t change the essential facts.
• Subtractive mixing is more complicated than my explanations might suggest, but the simplified theoretical rules for subtractive mixing provide a good starting point for predicting how colours will mix. For example, these rules predict that red and cyan will make black, but the complex aborption spectra of real dyes mean that a set of CMY printers inks will, in fact, produce some other dark colour.
• In 4-colour prints, some of the ink dots overlap and some of them sit side by side. The colour mixing isn’t purely subtractive—it’s a more complex process I call hybrid mixing. This has little affect on the hue but may reduce the saturation of the resulting colour.