This column was prompted by a query from a reader in Thessalonica seeking a good Photoshop technique for darkening weak skies, especially in black-and-white. After exploring a variety of tactics, we settled on the use of Selective Color adjustment layers. This is a hugely versatile command that receives rather short-shrift in most textbooks and Web discussions dealing with Photoshop. Given its versatility, it takes some time to understand its operation. Our approach was to explore its many options using a photograph of the familiar Macbeth ColorChecker Chart.
Go to the Layers palette, choose Create New Fill or Adjustment Layer, and click on Selective Color. This opens the menu seen in Figure 1. A choice must be made whether to invoke relative or absolute adjustments. Click on the Relative button. In the descriptions to follow, we work only with relative, as this ensures changes made to a color’s RGB values are in proportion to how strongly they are represented in that color. Changes made in absolute mode can be overwhelming, prompting undesirable changes in a color’s hue.
Creation of the actual layer is a two-step process: Select a color to operate on (reds, greens, blues, cyans, magentas, yellows, whites, neutrals, or blacks), then adjust the four sliders (cyan, magenta, yellow, black). The first step, color selection, is what makes the tool uniquely powerful. For example, if we select blues, any subsequent slider adjustments alter only image colors with a strong bluish cast. In the case of our color-checker photograph, the choice of blues allows us to alter the appearance of the chips identified as blue sky, blue f lower, purplish blue, blue, and cyan. The other 19 chips remain absolutely impervious to any slider adjustments. Had we selected greens as the operative color, we would be free to alter only the chips labeled foliage, bluish green, yellow green, and green.
With the color selected, adjustments are made by moving the sliders. All colors contain R, G, and B components. Moving the cyan slider changes only the selected colors’ R component; magenta alters only G, and yellow only B. The black slider is a special case and a useful shortcut. Setting this to, for instance, +50%, has exactly the same effect as setting the other three sliders to +50%.
So how to darken a sky? Open an adjustment layer, select relative mode, select blues, and shove the cyan, magenta and black sliders all the way to the right (+100%, or less if desired). Now select cyans and make the same three slider adjustments. Flatten the image, convert to Grayscale, and voila!—a greatly darkened sky without revision to other colors (Figure 2). Not dark enough? Add another Selective Color adjustment layer. The comparison in Figure 2 demonstrates the consequences of a single adjustment layer as just described relative to the original photograph (with thumbnails of the color versions).
We have focused on skies in response to the original query, but the same versatile tool may be used to selectively lighten foliage, improve tonality of furniture, achieve an old “ortho look” to portraits (for men at least; the grey-skinned, dark-lipped look is not generally flattering on women), and so on—all the while preserving the appearance of other colors.