Color Is Different

The concepts and adjustments we talk about with Curves apply equally to grayscale and color images. But when you work on a color image, you have to be more careful; otherwise, you might end up shifting the colors, rather than just the brightness, of your image. There are two ways to apply Curves to your image, and therefore two methods for limiting its effect on the brightness of a color image. First, you can apply Curves to the currently active layer by choosing Image > Adjustments > Curves. Immediately after applying Curves, you can choose Edit > Fade Curves and set the Mode pop-up menu to Luminosity (Figure 6.64). The Fade command will limit the last change you made (Curves, in our case) to changing only the brightness (luminosity is just another word for brightness) of the image—it will not be able to shift the colors or change how saturated they are.

Your other choice would be to apply Curves to more than one layer by choosing Layer > New Adjustment Layer > Curves. Then, when prompted (Figure 6.65), you would set the Mode pop-up menu to Luminosity. An adjustment layer will affect all the layers below it but none of the layers above it. It's also a nonpermanent change, because you can double-click on the adjustment-layer thumbnail in that layer to reopen the Curves dialog box and make changes. That means that any Curves techniques you use for adjusting grayscale images will work on color images if you use the Luminosity blending mode (Figures 6.66 to 6.68).

Color shifts aren't the only problems you'll encounter when adjusting color images with Curves. The mode your image is in might be having an adverse effect on the adjustment. RGB color images are made from three components (red, green, and blue). A bright green color might be made out of 0 red, 255 green, and 128 blue. When you first open the Curves dialog box, the pop-up menu at the top of the dialog box will be set to RGB, which will cause any points to affect the exact same R, G, and B values. Clicking on that green color in your image will display a circle at 165 on the curve, which will affect all the areas that contain 165 red, 165 blue, and 165 green. Equal amounts of R, G, and B create gray. So, simply clicking on the curve of a color image will usually cause the colors to shift in an unsatisfactory way, because the circle that appears when clicking on your image will not accurately target the area you clicked on. While working in RGB mode, all color areas will shift because their RGB mix will change as the Curves dialog box shifts the RGB values in equal amounts. Ideally it would affect only the exact mix of RGB that the color is made from, but it doesn't work that way in RGB mode. The solution to this problem is to convert your image to Lab mode by choosing Image > Mode > Lab Color. In Lab mode, your image is made from three components: Lightness, A, and B. When you adjust your image, the Curves dialog box will automatically set itself to work on the Lightness information, which will prevent your adjustment from shifting the color of your image and will also make it so that the circle will show up in the correct position to make accurate adjustments. Once you're done with your adjustment, I'd suggest that you convert the image back to RGB mode because many of Photoshop's features are not available in Lab mode. I don't use Lab mode for every color image; I reserve it for those images that are troublesome in RGB mode.