Applying colors to artwork is a common Adobe Illustrator task, and one that requires some knowledge of color models and color modes. When applying color to artwork, keep in mind the final medium in which the artwork will be published, so that you can use the correct color model and color definitions. Experimenting and applying color is easy using the feature-rich Swatches panel, Color Guide panel, and Edit Colors/Recolor Artwork dialog box in Illustrator.

Color models describe the colors we see and work with in digital graphics. Each color model, such as RGB, CMYK, or HSB, represents a different method for describing and classifying color. Color models use numeric values to represent the visible spectrum of color. A color space is a variant of a color model and has a specific gamut (range) of colors. For example, within the RGB color model are a number of color spaces: Adobe® RGB, sRGB, and Apple® RGB. While each of these color spaces defines color using the same three axes (R, G, and B), their gamuts are different.

About colors in digital graphics

We use color models to describe the colors we see and work with in digital graphics. Each color model, such as RGB, CMYK, or HSB, represents a different method for describing and classifying color. Color models use numeric values to represent the visible spectrum of color. A color space is a variant of a color model and has a specific gamut (range) of colors. For example, within the RGB color model are a number of color spaces: Adobe® RGB, sRGB, and Apple® RGB. While each of these color spaces defines color using the same three axes (R, G, and B), their gamuts are different.

When you work with the colors in a graphic, you are actually adjusting numerical values in the file. It’s easy to think of a number as a color, but these numerical values are not absolute colors in themselves—they only have a color meaning within the color space of the device that is producing the color.

Because each device has its own color space, it can reproduce colors only in its gamut. When an image moves from one device to another, image colors may change because each device interprets the RGB or CMYK values according to its own color space. For example, it is impossible for all the colors viewed on a monitor to be identically matched in a print from a desktop printer. A printer operates in a CMYK color space, and a monitor operates in an RGB color space. Their gamuts are different. Some colors produced by inks cannot be displayed on a monitor, and some colors that can be displayed on a monitor cannot be reproduced using inks on paper.

Even though it is impossible to perfectly match all colors on different devices, you can use color management to ensure that most colors are the same or similar enough so they appear consistent.

RGB, CMYK, HSB, and Lab color models

RGB

A large percentage of the visible spectrum can be represented by mixing red, green, and blue (RGB) colored light in various proportions and intensities. Where the colors overlap, they create cyan, magenta, and yellow.

RGB colors are called additive colors because you create white by adding R, G, and B together—that is, all light is reflected back to the eye. Additive colors are used for lighting, television, and computer monitors. Your monitor, for example, creates color by emitting light through red, green, and blue phosphors.

Additive colors (RGB)
Additive colors (RGB)

A. Red B. Green C. Blue 

You can work with color values using the RGB color mode, which is based on the RGB color model. In RGB mode, each of the RGB components can use a value ranging from 0 (black) to 255 (white). For example, a bright red color might have an R value of 246, a G value of 20, and a B value of 50. When the values of all three components are equal, the result is a shade of gray. When the value of all components is 255, the result is pure white; when all components have values of 0, the result is pure black.

Illustrator also includes a modified RGB color mode called Web Safe RGB, which includes only those RGB colors that are appropriate for use on the web.

CMYK

Whereas the RGB model depends on a light source to create color, the CMYK model is based on the light-absorbing quality of ink printed on paper. As white light strikes translucent inks, a portion of the spectrum is absorbed. Color that is not absorbed is reflected back to your eye.

Combining pure cyan (C), magenta (M), and yellow (Y) pigments would result in black by absorbing, or subtracting, all colors. For this reason they are called subtractive colors. Black (K) ink is added for better shadow density. (The letter K came into use because black is the “key” color for registering other colors, and because the letter B also stands for blue.) Combining these inks to reproduce color is called four-color process printing.

Subtractive colors (CMYK)
Subtractive colors (CMYK)

A. Cyan B. Magenta C. Yellow D. Black 

You can work with color values using the CMYK color mode, which is based on the CMYK color model. In CMYK mode, each of the CMYK process inks can use a value ranging from 0 to 100%. The lightest colors are assigned small percentages of process ink colors; darker colors have higher percentage values. For example, a bright red might contain 2% cyan, 93% magenta, 90% yellow, and 0% black. In CMYK objects, low ink percentages are closer to white, and high ink percentages are closer to black.

Use CMYK when preparing a document to be printed using process inks.

HSB

Based on the human perception of color, the HSB model describes three fundamental characteristics of color:

Hue

Color reflected from or transmitted through an object. It is measured as a location on the standard color wheel, expressed as a degree between 0° and 360°. In common use, hue is identified by the name of the color, such as red, orange, or green.

Saturation

Strength or purity of the color (sometimes called chroma). Saturation represents the amount of gray in proportion to the hue, measured as a percentage from 0% (gray) to 100% (fully saturated). On the standard color wheel, saturation increases from the center to the edge.

Brightness

Relative lightness or darkness of the color, usually measured as a percentage from 0% (black) to 100% (white).

HSB color model
HSB color model

A. Hue B. Saturation C. Brightness 

Lab

The CIE Lab color model is based on the human perception of color. It is one of several color models produced by the Commission Internationale d’Eclairage (CIE), an organization dedicated to creating standards for all aspects of light.

The numeric values in Lab describe all the colors that a person with normal vision sees. Because Lab describes how a color looks rather than how much of a particular colorant is needed for a device (such as a monitor, desktop printer, or digital camera) to produce colors, Lab is considered to be a device-independent color model. Color management systems use Lab as a color reference to predictably transform a color from one color space to another color space.

In Illustrator, you can use the Lab model to create, display, and output spot color swatches. However, you cannot create documents in Lab mode.

Grayscale

Grayscale uses tints of black to represent an object. Every grayscale object has a brightness value ranging from 0% (white) to 100% (black). Images produced using black-and-white or grayscale scanners are typically displayed in grayscale.

Grayscale also lets you convert color artwork to high-quality black-and-white artwork. In this case, Adobe Illustrator discards all color information in the original artwork; the gray levels (shades) of the converted objects represent the luminosity of the original objects.

When you convert grayscale objects to RGB, the color values for each object are assigned that object’s previous gray value. You can also convert a grayscale object to a CMYK object.

Color spaces and gamuts

A color space is a range of colors in the visible spectrum. A color space can also be a variant of a color model. Adobe RGB, Apple RGB, and sRGB are examples of different color spaces based on the same color model.

Gamuts of different color spaces
Gamuts of different color spaces

A. Visual gamut B. RGB color space C. CMYK color space 

The range of color encompassed by a color space is called a gamut. The different devices (computer monitor, scanner, desktop printer, printing press, digital camera) throughout your workflow operate within different color spaces and each with different gamuts. Some colors within the gamut of your computer monitor are not within the gamut of your inkjet printer, and vice versa. When a color cannot be produced on a device, it’s considered to be outside the color space of that particular device. In other words, the color is out of gamut.

About spot and process colors

You can designate colors as either spot or process color types, which correspond to the two main ink types used in commercial printing. In the Swatches panel, you can identify the color type of a color using icons that appear next to the name of the color.

When applying color to paths and frames, keep in mind the final medium in which the artwork will be published, so that you apply color using the most appropriate color mode.

Note:

If your color workflow involves transferring documents among devices, you may want to use a color-management system (CMS) to help maintain and regulate colors throughout the process.

About spot colors

A spot color is a special premixed ink that is used instead of, or in addition to, process inks, and that requires its own printing plate on a printing press. Use spot color when few colors are specified and color accuracy is critical. Spot color inks can accurately reproduce colors that are outside the gamut of process colors. However, the exact appearance of the printed spot color is determined by the combination of the ink as mixed by the commercial printer and the paper it’s printed on, not by color values you specify or by color management. When you specify spot color values, you’re describing the simulated appearance of the color for your monitor and composite printer only (subject to the gamut limitations of those devices).

Keep the following guidelines in mind when specifying a spot color:

  • For best results in printed documents, specify a spot color from a color-matching system supported by your commercial printer. Several color-matching system libraries are included with the software.

  • Minimize the number of spot colors you use. Each spot color you create will generate an additional spot color printing plate for a printing press, increasing your printing costs. If you think you might require more than four colors, consider printing your document using process colors.

  • If an object contains spot colors and overlaps another object containing transparency, undesirable results may occur when exporting to EPS format, when converting spot colors to process colors using the Print dialog box, or when creating color separations in an application other than Illustrator or InDesign. For best results, use the Flattener Preview or the Separations Preview to soft proof the effects of flattening transparency before printing. In addition, you can convert the spot colors to process colors by using the Ink Manager in InDesign before printing or exporting.

  • You can use a spot color printing plate to apply a varnish over areas of a process color job. In this case, your print job would use a total of five inks—four process inks and one spot varnish.

About process colors

A process color is printed using a combination of the four standard process inks: cyan, magenta, yellow, and black (CMYK). Use process colors when a job requires so many colors that using individual spot inks would be expensive or impractical, as when printing color photographs.

Keep the following guidelines in mind when specifying a process color:

  • For best results in a high-quality printed document, specify process colors using CMYK values printed in process color reference charts, such as those available from a commercial printer.

  • The final color values of a process color are its values in CMYK, so if you specify a process color using RGB (or LAB, in InDesign), those color values will be converted to CMYK when you print color separations. These conversions differ based on your color-management settings and document profile.

  • Don’t specify a process color based on how it looks on your monitor, unless you are sure you have set up a color-management system properly, and you understand its limitations for previewing color.

  • Avoid using process colors in documents intended for online viewing only, because CMYK has a smaller color gamut than that of a typical monitor.

  • Illustrator and InDesign let you specify a process color as either global or non-global. In Illustrator, global process colors remain linked to a swatch in the Swatches panel, so that if you modify the swatch of a global process color, all objects using that color are updated. Non-global process colors do not automatically update throughout the document when the color is edited. Process colors are non-global by default. In InDesign, when you apply a swatch to objects, the swatch is automatically applied as a global process color. Non-global swatches are unnamed colors, which you can edit in the Color panel.

Note:

Global and non-global process colors only affect how a particular color is applied to objects, never how colors separate or behave when you move them between applications.

Using spot and process colors together

Sometimes it’s practical to use process and spot inks in the same job. For example, you might use one spot ink to print the exact color of a company logo on the same pages of an annual report where photographs are reproduced using process color. You can also use a spot color printing plate to apply a varnish over areas of a process color job. In both cases, your print job would use a total of five inks—four process inks and one spot ink or varnish.

In InDesign, you can mix process and spot colors together to create mixed ink colors.

Comparing colors in InDesign and Illustrator

Adobe InDesign and Adobe Illustrator use slightly different methods for applying named colors. Illustrator lets you specify a named color as either global or nonglobal, and InDesign treats all unnamed colors as nonglobal, process colors.

The InDesign equivalents to global colors are swatches. Swatches make it easier to modify color schemes without having to locate and adjust each individual object. This is especially useful in standardized, production-driven documents like magazines. Because InDesign colors are linked to swatches in the Swatches panel, any change to a swatch affects all objects to which a color is applied.

The InDesign equivalents to nonglobal swatches are unnamed colors. Unnamed colors do not appear in the Swatches panel, and they do not automatically update throughout the document when the color is edited in the Color panel. You can, however, add an unnamed color to the Swatches panel later.

Named and unnamed colors only affect how a particular color updates in your document, never how colors separate or behave when you move them between applications.

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