The subtractive primary colors used by commercial printers — cyan, magenta, and yellow — are for the most part very light. Cyan absorbs only red light, magenta absorbs only green light, and yellow absorbs only blue light. On their own, these colors unfortunately don't do a good job of producing dark colors. In fact, at full intensities, cyan, magenta, and yellow all mixed together don't get much beyond a muddy brown. That's where black comes in. Black helps to accentuate shadows, deepen dark colors, and, of course, print real blacks.
In case you're wondering how colors mix in the CMYK model, it's basically the opposite of the RGB model. Because pigments are not as pure as primary colors in the additive model, though, some differences exist:
♦ Cyan and magenta: Full-intensity cyan and magenta mix to form a deep blue with a little violet. Subtract some cyan to make purple; subtract some magenta to make a dull medium blue. All these colors assume a complete lack of yellow.
♦ Magenta and yellow: Full-intensity magenta and yellow mix to form a brilliant red. Subtract some magenta to make vivid orange; subtract some yellow to make rose. All these colors assume a complete lack of cyan.
♦ Yellow and cyan: Full-intensity yellow and cyan mix to form a bright green with a hint of blue. Subtract some yellow to make a deep teal; subtract some cyan to make chartreuse. All these colors assume a complete lack of magenta.
♦ Cyan, magenta, and yellow: Full-intensity cyan, magenta, and yellow mix to form a muddy brown.
♦ Black: Black pigmentation added to any other pigment darkens the color.
♦ No pigment: No pigmentation results in white (assuming white is the paper color).
If you're used to editing RGB images, editing in the CMYK mode can require some new approaches, especially when editing individual color channels. When you view a single color channel in the RGB mode (as discussed in the following chapter), white indicates high-intensity color, and black indicates low-intensity color. It's the opposite in CMYK. When you view an individual color channel, black means high-intensity color, and white means low-intensity color.
This doesn't mean RGB and CMYK color channels look like inverted versions of each other. In fact, because the color theory is inverted, they look much the same. But if you're trying to achieve the full-intensity colors mentioned in the preceding section, you should apply black to the individual color channels, not white as you would in the RGB mode.
Should I edit in CMYK?
RGB doesn't accurately represent the colors you get when you print an image because the RGB color space contains many colors — particularly very bright colors — that CMYK can't touch. This is why when you switch from RGB to CMYK, the colors appear duller. (If you're familiar with painting, RGB is like oils and CMYK is like acrylics. The latter lacks the depth of color provided by the former.)
For this reason, many folks advocate working exclusively in the CMYK mode, but I do not. Although working in CMYK eliminates color disappointments, it is also much slower because Photoshop has to convert CMYK values to your RGB screen on the fly.
Furthermore, your scanner and monitor are RGB devices. No matter how you work, a translation from RGB to CMYK color space must occur at some time. If you pay the extra bucks to purchase a commercial drum scan, for example, you simply make the translation at the beginning of the process — Scitex has no option but to use RGB sensors internally — rather than at the end. Every color device on Earth, in fact, is RGB except the printer.
You should wait to convert to the CMYK mode until right before you print. After your artwork is finalized, choose Image ^ Mode ^ CMYK and make whatever edits you deem necessary. For example, you might want to introduce a few color corrections, apply some sharpening, and even retouch a few details by hand. Photoshop applies your changes more slowly in the CMYK mode, but at least you're only slowed down at the end of the job, not throughout the entire process.
Before converting an image to the CMYK color space, make certain Photoshop is aware of the monitor you're using and the printer you intend to use. These two items can have a pronounced effect on how Photoshop generates a CMYK image. I discuss how to set up your personal RGB and CMYK color spaces in the "Creating Color Separations" section of Chapter 18.
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