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If this were a perfect world, cyan, magenta, and yellow would
be all the colors you’d need to print an RGB image. But alas,
the world is perfect only in its lack of perfection. Thus, it’s
the job of the key color, black, to set the world straight. In
fact, you could argue that the fourth channel, Black, is the
most important ink in CMYK output.
In the world of RGB, black exists naturally; in CMYK, it has
to be generated. Let’s start with RGB. Here, black is the de-
fault color, the one that appears if no other color is present.
As pixels are turned on, the black goes away. As illustrated
in the figure directly below, light pixels in the Red channel
add red, those in the Green channel add green, and those in
the Blue channel add blue. Any one channel is very dark on
its own; only by combining channels can they overcome the
darkness, as the middle and bottom rows of the figure show.
By contrast, color printing is about overcoming the lightness.
In the collection of images below this column, we see the re-
sults of converting the RGB channels from the previous figure
into cyan, magenta, yellow, and black using the CMYK profile
that you created in “Preparing a CMYK File for Commercial
Reproduction” (page 484). Assuming you output to white
paper, the default color of printing is white, and therefore it’s
the job of the inks to make the white go away. The channels
start out very light, as in the top row of the figure; but as one
layer of ink mixes with another, the photograph becomes
progressively darker, as in the middle and bottom rows. (To
help you navigate through the figure, samples with three or
more inks include a black outline.)
The CMY channels are derived in large part from their
RGB complements. But because every printer subscribes toWhy (and How) Three Channels Become four
Red Green BlueRed + Green Green + Blue Blue + RedRed + Green + BlueCyan (C) Magenta (M) Yellow (Y) Black (K)C+M M+Y C+Y C+KC+M+K (no Y) M+Y+K (no C) C+Y+K (no M) M+KC+M+Y (no K) C+M+Y+K Y+K440 Lesson 12: Print and Web Output