3. Managing Colors

This chapter presents not only color management theory, but how color tools are used to create and edit graphics in GIMP.

Managing colors in GIMP is a lot more than just working with colors. Color needs to be managed for a variety of reasons, including accounting for how an external device, such as a digital camera, manages color, and setting color characteristics in GIMP for printing. Managing colors can mean creating different profiles for a variety of color needs, working with different color models, such as RGB, HSV, and CMYK, adjusting color using curves and levels, and a great deal more. This chapter explores the expansive frontier of color in GIMP.

It would be nice and convenient to have one color profile or color model fit all our needs, but it just isn’t so. The color model you use to create and edit an image depends on the purpose of the image. For instance, you may use the RGB (red, green, blue) color profile to manage color on a digital image, but you can’t tell exactly how the photo will appear if printed. You usually manage an image you intend to print using the CMYK (cyan, magenta, yellow, key/black) color model. HSV (hue, saturation, value) is similar to RGB, but each element doesn’t represent a separate color.

In this chapter, you’ll learn how to set Color Management in GIMP preferences, review and select the correct color model for specific jobs, gain the ability to set colors in different color models, successfully work with color decomposition and managing an image’s background color.

Color Management Basics

Before going into creating and editing images, you’ll first have the opportunity to understand the basics of managing color profiles using GIMP. If you’ve ever viewed the same photo from two different computers, and especially from a PC and a Mac, you probably have noticed that the photo looked different—maybe dramatically different—from one device to another. You can use color profiles in GIMP to compensate for these differences, at least somewhat.

Color profiles are files that represent the differences between the standard or baseline color of an image and the way devices such as digital cameras, printers, and scanners characterize color. GIMP uses its own built-in profile, which should be sufficient for most tasks, but it’s helpful to become familiar with what else is available.

Opening Color Management Preferences

You can see the different features that can be modified in GIMP color management, but you won’t be able to take advantage of all of them, at least as a beginning user. This will make more sense after you’ve seen Color Management Preferences. If you don’t have GIMP open already, open it now. On the Image Window, click Edit and then click Preferences. When the Preference dialog opens, click Color Management in the left pane.

Reviewing the Color Management Preferences Dialog

On the Color Management Preferences dialog, various drop-down menus enable you to choose how color management is performed in GIMP, as shown in Figure 3.1.

Figure 3.1. The Color Management selection in the Preferences dialog offers methods of adjusting GIMP color management.

This dialog allows you to tell GIMP how you want color profiles to be managed, including changing the default color profiles for RGB, CMYK, and the monitor color profile, previewing how an image will appear when printed, and setting all pixels that would not normally print to a special color.

• The Mode of Operation drop-down menu offers three selections:

• Color Managed Display—This is the default selection; it provides completely corrected images related to whatever color profile is operating.

• No Color Management—This completely turns off GIMP’s color management feature.

• Print Simulation—This choice not only applies the selected color profile but also the print simulation profile, allowing you to preview images as they will appear when printed.

• The RGB Profile, CMYK Profile, Monitor Profile, and Print Simulation Profile drop-down menus consist of two selections:

• None (the default)

• Select Color Profile from Disk

• When you select the Try to Use the System Monitor Profile check box, GIMP attempts to use the default profile of your computer’s monitor.

• The Displaying Rendering Intent and Softproof Rendering Intent drop-down menus offer these four options:

• Perceptual—This rendering intent, typically used with photos, fits one gamut into another while preserving the relative position of all colors to each other. This method is less accurate than other methods if you need precise color matching, but it is fine for casual use.

• Relative Colorimetric—This rendering intent leaves colors inside the gamut unchanged, but colors outside the gamut are converted to the closest reproducible hue, usually with reduced saturation. This is more accurate than Perceptual rendering because it preserves more of the original colors. This method remaps the native white point color in the source file to the white point of the destination device.

• Saturation—Less accurate, but more vivid. Often used for business graphics, this rendering intent converts saturated colors in the source graphic gamut to the closest saturated colors available in the destination gamut.

• Absolute Colorimetric—Used largely for proofing, this rendering intent maintains the native device white point of the source image. In color reproduction, a gamut is a particular subset of colors used for a specific purpose, such as accurately representing colors for a color space or for a device such as a printer.

When you select the Mark Out Gamut Colors check box, all pixels that are not a printable color are marked as a special color, which you choose by clicking the button to the right to open the Select Warning Color editor, as seen in Figure 3.2.

Figure 3.2. The Select Warning Color editor lets you choose what color to make pixels that will otherwise not print.

The File Open Behavior drop-down menu lets you select the type of behavior you want from color management when you open an image:

• Ask What to Do—The default behavior.

• Keep Embedded Profile—This will retain the color profile already in the image, rather than remapping it to the color workspace.

• Convert to RGB workspace—This will remap the colors in the image you’re opening to the colors you’ve selected for your workspace.

Usually, color profiles for specific devices are created by the device manufacturer. To be used by all color editing platforms, including GIMP, color profiles are created using the International Color Consortium ICC-profile standard. Color profiles are stored in files with an .ICC extension and can be embedded within images. GIMP uses a single sRGB color profile, which is sufficient for most common tasks.

Understanding Color Models

In Chapter 1, “What Is Gimp?” you explored the very basics of the GIMP color editor, but there’s a lot more to learn. Also, learning about the color editor is a good way to understand how different color models work, as well as the different ways you can manipulate those models.

Before you work with the RGB, HSV, and CMYK color models, it will be helpful if you gain a greater understanding of what they are and how they’re used.

Understanding RGB

As you learned in the Color Management Basics section of this chapter, Standard RGB, or sRGB, is the only color profile installed in GIMP by default. sRGB is a variant of the RGB color space and was created cooperatively by HP and Microsoft.

Understanding RGB can help you understand color as it’s used on the Web. 24-bit RGB is represented by three integers that can contain values between 0 and 255. As you can see in Figure 3.3, in the Change Foreground Color dialog, the values 255, 0, 0 produce the color red (R = 255, G = 0, B = 0).

Figure 3.3. The RGB sliders are used to set different color values.

If you look at the HTML notation field in Figure 3.5, you’ll see that although red is rendered in RGB decimal notation as 255, 0, 0, the HTML notation is ff0000. This is the hexadecimal value for the color red. Hexadecimal is a base-16 numbering system and is commonly used to indicate colors on websites.

If you want to set the Change Foreground Color dialog to a specific color on your screen but don’t know its decimal or hexadecimal value, use the Color Chooser tool. In the Change Foreground Color dialog, click the Color Chooser tool shown in Figure 3.4, and then click a color anywhere on your screen. The color in the dialog will become that color and the numerical values will be changed accordingly.

Figure 3.4. The Color Chooser lets you select any color on your computer screen and set the Change Foreground Color dialog to the same color.

Understanding HSV

Like RGB, the HSV model uses three numeric values to represent a color; however, those values don’t represent separate colors but rather conceptual properties of a given image. Also, whereas the values for R, G, and B all fall within the 0 to 255 range; the ranges for HSV values are not uniform.

HSV stands for Hue, Saturation, and Value. On the Image Window toolbar, if you click Colors, Hue-Saturation, you’ll open the Hue-Saturation dialog, which is the most common tool people use to edit these qualities in a graphic. On the Change Foreground Color dialog shown in Figure 3.5, you can see that the color red is represented as 0, 100, 100 (H = 0, S = 100, V = 100), but what does this mean?

Figure 3.5. The HSV color model modifies color based on conceptual image properties.

Hue is the only value you can adjust that actually represents a color. The range of values for HUE is 0 to 360. Imagine this to mean degrees on a circle. If you start at 0 and go all the way around to 360, you arrive at the same point on the circle.

The Hue “continuum” is a spectrum that goes from red tones to orange tones to yellow to green to blue to magenta and then back to red.

Saturation is how “intense” you want the color to appear. The range of values is 0 to 100, with 0 representing a washed out or pale intensity, to 100, which is the maximum intensity the color can attain.

Value also has a range that goes from 0 to 100 and is the brightness of a specific pixel. For example, the color black has a value of 0, whereas a very bright pixel (which doesn’t have to be white) has a value of 100.

You are more likely to use the RGB color model when you are thinking of colors as absolute values. You are more likely to use the HSV model when you are thinking of colors as how they appeal to you intuitively. This is like trying to select paint for the walls of your kitchen by looking at color swatches. You may ask yourself, “Does this need to have more blue or more green?” When you are selecting colors based on color tone, shade, or intensity, the HSV model is a better tool.

The Triangle Selector tab in the Change Foreground Color dialog is an alternative way to edit colors using the HSV model. The ring in this tool represents the Hue, because a circle has 360 degrees. Dragging your cursor around the circle changes the Hue value, and dragging your cursor inside the triangle changes both the Saturation and Value.

Remember, you will tend to use this model when you are selecting colors by “feel.” Trying to select specific numeric Saturation and Value numbers by dragging the cursor in the triangle is fairly difficult because the scales for those qualities aren’t equal.

Understanding CMYK

The CMYK color model is used by printers, and the acronym stands for ink colors Cyan, Magenta, Yellow, and Key. Key isn’t a color, but the color black is considered the key because four-color printing plates are thought of as “keyed” to the color black. If you’ve ever worked with any color laser printers or commercial offset printers, you know that each of these four colors has its own color cartridge (or ink well) within the machine.

Most people who print images from their computers don’t give much thought to color models, but professional artists and photographers give it a great deal of thought (because they are usually using CYMK devices for output) and use the CMYK color model for that reason.

The CMYK values range is from 0 to 100 percent, with 0 percent meaning that the white canvas is completely untouched by the selected color and 100 percent meaning that it’s completely covered. The color red, which we’ve been using as an example when describing other color models, is represented by 0, 100, 100, 0 (C = 0, M = 100, Y = 100, K = 0).

Despite the fact that the Change Foreground Color dialog has a CMYK tab, and values for each color can be set on that tab, GIMP 2.6 does not support CMYK colors internally. If you set CMYK values, GIMP converts the color to RGB for its own use and prints from RGB. If you have installed a color profile for your printer, GIMP will use it on the CMYK tab.

For most home and even many business uses, RGB is more than sufficient. If you need to work with CMYK color in a graphic, but lack the professional know-how in this area, you can provide the digital image to a professional printing house, and they can make the conversion for you.

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Choosing Color Models

The color editor allows you to choose different color models to be used for various purposes. If you’re used to working with tabbed interfaces, such as the Firefox web browser, you’ll be right at home using the tabbed color model selectors.

With GIMP open, on the Main toolbar, click the Foreground Color Editor selector to open the Change Foreground Color dialog.

1. With the Change Foreground Color dialog open, click the GIMP tab (the tab with a picture of Wilber on it, as shown in Figure 3.6).
   

   Figure 3.6. The RGB tab on the Change Foreground Color dialog is the default color selection.

   

2. Click inside the main color pane and drag your mouse to observe how you can change colors.
3. Click inside the color bar to the immediate right of the color pane and drag the mouse up and down to observe how you can change colors.
4. Click the Reset button to return the colors to their default values.
5. Click the CMYK tab (the tab with the printer on it).
6. Experiment with the sliders in the available pane to discover how you can work with the CMYK color model, as shown in Figure 3.7.
   

   Figure 3.7. The CMYK tab on the Change Foreground Color dialog uses sliders to let you manage colors from the CMYK model.

   

7. Click the Watercolor tab (the tab with the paintbrush on it).
8. Right-click in the Watercolor main pane and then left-click anywhere in the pane to adjust the color.
9. On the Image Window, click File, New, and in the Create New Image dialog, click OK.
10. Select the Paintbrush tool in the Main Toolbox, and then click around in the blank canvas to see the effect.
11. Close the canvas without saving it.
12. Click the Main Toolbox to make it the active window and then press the D key on your keyboard to return the colors to their default values.
13. Open the Change Foreground Color dialog again and click the Triangle Selector tab.
14. Click anywhere in the triangle and drag your mouse to adjust the color.
15. Click in the circle and use your mouse to drag the triangle, adjusting the color.
16. Click the Reset button to restore the default values.
17. Click the Palette Selector tab.
18. Click any of the color palettes to adjust the color, and then click any of the grayscale palettes to adjust the gray tones.
19. Click the Reset button and then click the GIMP tab.

Editing Colors and Tones

At one time, a photographer could edit a photograph in only two venues: the camera and the darkroom. Because most photography now is digital, the tools of the darkroom are becoming a lost art, but new tools are now available to take their place. Taking a digital photograph isn’t an exact science and sometimes unanticipated results occur. When those results affect the color, shading, or tones of the photo, GIMP provides an ample assortment of methods to help you set things right.

Working with Color Balance

There are probably times when you see a particularly beautiful sunset or an amazingly picturesque landscape and want to capture it on film, or rather, on a memory card. However, a difference can exist between what your eyes see and how your brain interprets the image, and how your camera actually records the view. This sometimes doesn’t become apparent until you see an enlarged image of the photo on your computer (the camera’s viewfinder is often too limited to show many details).

Many circumstances can affect how color is recorded in a photo differently from what you see with your eyes. Often the color of indoor photos can appear yellow because of the available lighting. Other factors can affect outdoor photos, including available lighting, exposure time, lens flare, and the like. GIMP has tools that can help you fix the color balance of such photographs.

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Using Color Balance

The obvious tool to modify an image’s color balance is the Color Balance tool. It’s hardly the only utility available in GIMP to perform the task, but it is the first one most people think of. The tool allows you to select the type of pixels you want to affect: Shadows for dark pixels, Midtones for medium pixels, and Highlights for light pixels. Then you adjust the sliders to affect colors along three continuums.

1. Navigate to a photo on your computer for which you want to adjust the color balance and open it with GIMP.
2. On the Image Window toolbar, click Tools, Color Tools, and then click Color Balance.
3. Select the option button for the type of pixels you want the tool to affect, such as Midtones.
4. Use the three sliders to adjust the color balance of the photo, moving the first one toward either cyan or red, the second toward either magenta or green, and the third toward either yellow or blue, as shown in Figure 3.8.
   

   Figure 3.8. The Color Balance dialog lets you modify the colors of an image using slider bars.

   

   The numeric values in the fields by each slider range from -100 to 100, with the default value of zero representing the original color of the image. The sliders let you increase the values of the RGB colors in one direction and the CMY colors

5. When you have adjusted the color balance to your satisfaction, click OK, and then save the image.

Depending on the effect you desire, you’ll likely need to experiment with the settings until you achieve an optimized image. If you want to use a particular setting again, you can save it by clicking the plus sign to the right of the Presets drop-down and, when the Add Settings to Favorites dialog appears as shown in Figure 3.9, give the settings a unique name in the available field. Then click OK.

Figure 3.9. Use the Add Settings to Favorites dialog to save settings configured in the Color Balance dialog.

To reset one specific slider to its default value (the original value of that color component in the image), select the slider and then click Reset Range. To reset all the sliders to the original values, click Reset.

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Using Hue-Saturation

Earlier in this chapter, you learned about the HSV color model, where an image’s color is adjusted by modifying the hue, saturation, and lightness of the image. The Hue-Saturation dialog lets you edit the color balance of a graphic using this model.

1. On your computer, navigate to a photo for which you want to adjust the color balance and open it with GIMP.
2. On the Image Window toolbar, click Tools, Color Tools, and then click Hue-Saturation.
3. If you want to adjust only a single color, select the option button for the RGB or CMY color in the Set Primary Color to Adjust area; otherwise, let the default Master setting remain to modify all colors with the sliders.
4. Use the Hue, Lightness, and Saturation sliders to adjust the color balance of the photo to your desired preference, as shown in Figure 3.10.
   

   Figure 3.10. The Hue-Saturation dialog lets you adjust an image’s color balance using the HSV color model.

   

5. When you are done, click OK and then save the image.

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Using Curves to Adjust Color Balance

Although the Curves tool can be used to modify an image’s contrast and transparency, it’s also excellent for editing color and brightness. However, it’s not as intuitive to use as some of the other GIMP color tools. The curve tool works on the active layer of an image and is considered a more advanced utility. At its core though, it adjusts RGB colors and is a very powerful option available to you.

1. On your computer, navigate to a photo for which you want to adjust the color balance and open it with GIMP.
2. On the Image Window toolbar, click Tools, Color Tools, and then click Curves.
3. To select a specific color channel, click the Channel menu and choose Red, Green, or Blue (for this example, choose Red).
   

   If your goal was to adjust brightness or contrast, you would accept the default Channel option of Value.

4. To the right of the Channel option are two icons that allow you to select either Linear Histogram (the default) or Logarithmic Histogram. Accept the default selection.
5. Under the chart pane, accept the default Curve Type of Smooth.
6. In the chart pane, click and drag the curve to adjust the color channel, watching for the changes in the preview image, as shown in Figure 3.11.
   

   Figure 3.11. The Curves dialog lets you adjust an image’s color balance by modifying color channels.

   

7. When the photo’s color channel has been edited to your satisfaction, click OK; then either save the image or close it without saving your changes.

If a particular color channel hasn’t been selected, changing the horizontal and vertical gradients by dragging the curve in the chart modifies the input and output tonal scales from shadows to highlights. Selecting a specific color channel and then dragging the curve adjusts the quality of the color selected, applying less or more of the color.

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Using Levels to Adjust Color Balance

The Levels tool works similarly to the Curves tool in that you can adjust color balance by selecting and modifying a specific color channel on the active layer. However, Levels can also modify all color channels simultaneously and is employed to edit a predominant color cast by using the White Point, Gray Point, and Black Point color dropper buttons.

1. On your computer, navigate to a photo for which you want to adjust the color balance and open it with GIMP.
2. On the Image Window toolbar, click Tools, Color Tools, and then click Levels.
3. To adjust the color cast of the photo, under All Channels select one of the eye-dropper buttons, such as Black Point, as shown in Figure 3.12, and then click on a dark portion of the photo.
   

   Figure 3.12. The Levels dialog lets you adjust an image’s color balance by selecting a color point.

   

4. Under Output Levels, use the arrows for the number fields on either side of the bar to adjust the color cast for the selected color.
   

   You may need to choose other eye dropper buttons under All Channels and select different portions of the image to view any significant changes. Also, all colors will be modified, regardless of the setting of the Channel button near the top of the dialog.

5. When you are finished, click the Reset button to return the photo to its original values.
6. To the left of the eye-dropper buttons, click the Auto button and observe the effect.
7. When you are satisfied with the result in the preview, click OK, then either save the image or close it without saving.

It takes time and practice to use the Levels utility with any sort of expertise, but a very quick and effective way to use it is by employing the Auto button. As you saw in the previous exercise, one click of the button can produce remarkable results. If you have a photo with a particular color cast that you want to remove or edit, try the Auto button in the Levels dialog to see if it solves the problem.

Working with Color Channels

GIMP uses Red, Green, and Blue color masks that show only those color components. As you know, GIMP’s native color model is the RGB model, so it’s easy to access and work with these colors as necessary to edit an image.

Viewing Color Channels

Any color image you are likely to open using GIMP is composed of separate Red, Green, and Blue color channels. Those channels together are what make up all the color in a photo or other graphic, but you can also see what an image looks like with one or more of those colors masked or removed.

Open an image in GIMP and then, on the Image Window toolbar, click Windows, Dockable Dialogs, and then click Channels. When the Channels dialog opens as shown in Figure 3.13, practice clicking the eye button next to each color to remove it from view, and then clicking it again to restore it.

Figure 3.13. The Channels dialog lets you see how colors in an image are displayed when one or more color channels are masked.

When you’re finished, close the Channels dialog.

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Making a Black and White Image with Color Decomposition

You may have an occasion where you want to make a color image appear as Black and White. GIMP has a number of tools that can easily produce this result, but sometimes the black and white image appears washed out or indistinct. Using the Decomposition tool lets you create a black and white photo by separating each RGB color channel into layers.

It may seem easier just to change the color mode to grayscale or just to desaturate the image, but they render the same effect. The Decomposition tool lets you choose the color model you desire, such as RGB, separates each color channel, and renders each channel in black and white. You can then select the channel that produces the desire effect.

1. Open the color photo you want to convert to black and white with GIMP.
2. On the Image Window toolbar, click Colors, Components, and then click Decompose.
3. When the Decompose dialog appears, under Extract Channels, if necessary, select RGB.
4. If necessary, clear the Decompose as Layers check box and then click OK.
5. When the duplicate black and white images of the original appear, review each image to determine which color channel renders the best black and white photo, as shown in Figure 3.14.
   

   Figure 3.14. The image is duplicated for each of the three color channels as black and white.

   

   Usually the green layer will render the most attractive black and white image, but it’s helpful to see each layer to verify this.

6. When you are satisfied with a particular black and white color image, save that image and discard the others.

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Changing a Background Color with Color to Alpha

In the exercises for both the Curves and Levels utilities, if you made a color channel selection (Red, Green, or Blue) you may have also seen a selection for Alpha. This selection may be unavailable in those menus because Alpha represents the transparent quality of the image. If the image has no transparent layer, the Alpha option isn’t accessible.

In Chapter 2, “Basic Editing,” you learned how to make the background of an image transparent by adding an Alpha channel to the image and then selecting the areas you wanted to make transparent with the Fuzzy Select tool. You can also perform a similar task by using Color to Alpha. For this exercise, select an image that has a uniformly colored background.

1. Open the desired image with GIMP.
2. On the Image Window toolbar, click Colors and the Color to Alpha.
3. When the Color to Alpha dialog opens, click the button (colored white by default) to the right of the word From.
4. When the Color to Alpha Color Picker dialog opens, use the Color Picker tool (eye dropper) to select the background of the image.
   

   This color picker tool is not the same one you see in the Main Toolbox palette and does not work with that tool. For this task, you can only use the color picker tool that is next to the HTML notation field in the Color Picker dialog.

5. Click OK on the Color to Alpha Color Picker dialog.
6. Click OK on the Color to Alpha dialog.
7. On the Main Toolbar, open the Change Foreground Color dialog and change the foreground color to something different than the original background color of the image.
8. On the Image Window toolbar, click Layer and then click New Layer.
9. When the New Layer dialog opens, accept the default name and, under Layer Fill Type, select Foreground Color, and then click OK.
10. On Toolbox Options, click the Layers tab and select the new layer by clicking it.
11. Click the Lower This Layer button, as shown in Figure 3.15, until the new background color is visible, and then save the image.
    

    Figure 3.15. The Channels dialog lets you see how colors in an image are displayed when one or more color channels are masked.

    

Color to Alpha makes transparent any color that’s selected by changing the color channel to an alpha channel. It’s then possible to substitute another color or even another image as a layer. You could have also selected the subject of the image with a selection tool, but sometimes that leaves rough edges that hold small bits of the original color, making the effect look less than professional.

The last exercise gave you an idea of what it’s like to work with Layers. Chapter 4, “Using Layers,” will give you much more experience.