color mapping

The Digital Eye: Mastering Color Mapping in Photoshop

Stephen Romaniello is an artist and educator teaching digital art at Pima Community College in Tucson, Arizona for over 20 years. He is a certified instructor in Adobe Photoshop and the author of several books on the creative use of digital graphics software. Steve is the founder of GlobalEye systems, a company that offers training and consulting in digital graphics software and creative imaging.

Color is one of the most significant factors in our experience of the world.

Because color plays such an important role in our lives, how we use it to communicate ideas is essential to the impact of our images.

This article explores features in Photoshop that radically alter colors. Color-mapping operations go beyond brightness and contrast adjustments and simple color fills, they provide the means to alter the basic characteristics of color while maintaining and even improving image detail. These features can potentially change the entire color scheme of a picture or create eye-popping graphic effects.

Just Adjust

Most of the color-mapping operations in Photoshop can be found under the Image > Adjustments submenu. Adjustments from this menu are destructive because they apply effects directly to the pixels on the targeted layer. It is therefore advisable to make an adjustment layer that attaches the data to a separate layer. Separating adjustment data to a layer enables access for further alterations and assures a non-linear workflow, something I always encourage. The Adjustments panel displays icons of all of Photoshop’s primary color altering features. When an icon is clicked an adjustment layer is automatically created. The Adjustments panel works in sync with the Properties panel. When an adjustment icon is selected the Properties panel displays its controls (see Figure 1).

Changing Hue and Saturation

Probably the most frequently used color mapping adjustment is Hue/Saturation. This panel controls the basic components that change color. Dragging the Hue slider changes relative color relationships, and can produce some really beautiful and unexpected color combinations.

It’s worth experimenting with Hue/Saturation because you’ll find that changing the reality of an everyday landscape into a brilliant fauvist work of art is fun and easy. A humdrum blue sky can be transformed into electric orange and the ordinary leaves on the trees can be changed to bubble-gum pink (see Figure 2).

Here’s how the panel works. By default, the values you see in the Hue/Saturation dialog box are a color’s position on the color wheel expressed in degrees from -180 degrees to 180 degrees, with 0 being the midpoint or the color as it exists before the alteration. Values in the box reflect the amount of rotation from the original color. Dragging the sliders to the right, or a positive value, indicates a clockwise rotation of the color wheel. Moving it to the left, to a negative value, indicates a counterclockwise rotation.

The color bars at the bottom of the dialog box are an indicator of how the colors change as you move the Hue slider. By default, the color bars are aligned as in Figure 1. As you move the Hue slider, the top color bar remains in place and represents the entire range of colors prior to the change. The bottom color bar is dynamic and moves as you drag the Hue slider and realigns with the colors on the top bar to reflect the relative change of colors as in Figure 3.

Targeting Color Ranges

The list at the top of the dialog box affects specific ranges of colors. The Master option permits the entire spectrum of color to be affected.  Each color in the list targets a specific range of colors and excludes other colors from the alteration. The colors are divided into ranges of 90 degrees each, including the overlap—the amount that the colors adjacent to the target color on the color wheel are affected. The list contains colors by name: Master, Reds, Yellows, Greens, Cyans, Blues, and Magentas.

When a specific color is selected from the list, the alteration is limited to colors within that range as in Figure 4. An adjustment slider appears between the two color bars, telling you which colors are being affected and letting you increase or decrease the range of hues affected by dragging the ramps to the left or right. The dark gray ramp encompasses the colors that are most affected. The light gray ramp indicates the overlap of adjacent colors that are less affected. The ramps can be dragged to the left or right to expand or decrease the range.

Limiting Colors

The overall number of colors in an image can be limited to produce gorgeous posterized effects. The Posterize command can be applied directly to an image with the Image > Adjustments > Posterize command, or with a Posterize adjustment layer. If the posterization is applied to an RGB image there is no direct way of controlling how the colors are mapped because the Posterize function applies the chosen quantity of colors to each channel of the image. For example, if you enter 2 while working in an RGB image, six colors are produced (two per channel) as in Figure 5. These effects can be interesting but with no way to control the color that is produced they can be unpredictable.

A better technique is to first convert the image to grayscale, and then convert it back to RGB. The image can then be posterized to the specific number of shades of gray. Each value of gray can be selected and then filled with the unique color. This effect simulates a multicolored serigraph (a type of screen-printed print).

Here is a detailed step-by-step illustrated workflow of the process (as shown in Figure 6):

  1. Open a full color RGB image (see Figure 6a).
  2. Choose Image > ModeGrayscale to convert the image to 256 gray values (see Figure 6b)
  3. Click the Posterize icon in the Adjustments panel and enter a Levels value of 4—you can enter any amount from 2 to 255 (see Figure 6c)
  4. Click OK. Notice that the image is now divided into four shades of gray: white, light gray, dark gray, and black (see Figure 6d)
  5. Choose Image >Mode > RGB to convert the image back to RGB.
  6. Make a new layer and place it at the top of the stack. Name it "Color."
  7. From the Tools panel, choose the Magic Wand tool. In the Options bar set the Tolerance to 1; clear the Anti-Aliased and Contiguous options and choose Sample All Layers. Click an area of white to select all of the white in the image.
  8. Choose Window > Show Swatches. Pick a light color from the Swatches panel. Target the Color layer and press Option + Delete (Mac) or Alt + Backspace (Windows) to fill the selected area with the color (see Figure 6e).
  9. Repeat the process for each of the additional shades of gray choosing a correspondingly darker color for each shade (see Figure 6f).

Gradient Maps

A gradient is a continuous blend of colors. Gradients can created and saved using the Gradient Editor (see Figure 7). A gradient map applies the colors of gradient to an image based on corresponding brightness values. In other words, light areas of the image are colored with the colors on the right side of the gradient and dark areas are colored with colors on the left side of the gradient and everything else in between is colored with the corresponding position of the gradient—from left to right—dark to light.

As you can see, this technique can radically alter the color scheme and produce pretty wild effects as shown in Figure 8.

The best way to apply this technique is with a Gradient Map adjustment layer. It’s helpful to display the Swatches panel so that you can quickly choose colors. When you choose the Gradient Map icon from the Adjustments panel, the Properties panel is displayed. Click on the gradient ramp to display the Gradient Editor. Click on a house shaped color stop and choose a color from the Swatches panel. Repeat the process for additional colors of the gradient. As you do, your image changes on the fly to reflect the results.

Radical, Never Boring

I hope you find these color mapping techniques useful and entertaining. Images don’t have to always look boringly real. The leaves on the trees don’t always have to be green, the sky can be any color you choose. Color mapping techniques can radically modify the look of your image to be totally different and unique. I encourage you to experiment and play with different color combinations to achieve the perfect effect for your piece.