The Hidden Power of Adjustment Layers in Adobe® Photoshop®: Color Correction Curves
This image has good contrast and brightness, but a few areas still need to be cleaned up. Ordinarily, using Curves adjusts the luminosity or apparent brightness along with everything else. Even though you could alter the Red, Green, and Blue channels separately in the Curves panel, messing with those can cause changes in brightness that you won’t want.
The solution here is to change the Curves layer’s blending mode to Color, giving you control over each of the channels without changing anything else about the picture.
Look at the Curves properties panel. The upper-right of the graph affects highlights. With the Red channel chosen from the drop-down box, sliding the top control point to the left increases red saturation levels. Dragging down the right-side control point removes red. Dragging the lower left-corner control point does the opposite: Moving it up along the left increases red in the shadows, and moving it to the right along the bottom removes red from the shadows.
For the Red and Blue channels, I want to increase saturation in the highlights, but leave the shadows more or less alone. Both of these channels, selected from the Curves panel drop-down menu, get a little boost by moving the upper-right control point over to the left along the top. But green is a little more pronounced in the shadows, so I’ll move the lower-left control point to the right along the bottom.
When I adjust the Green channel’s curve, I try to make sure that the axis line goes through the center of the graph to avoid introducing an overall green cast. The Red and Blue channels were lacking slightly, so moving them above the center line works to adjust the overall color range the way I want it.
Of course, you can push this technique into special effects by twisting the curves to the extreme. But for simple corrections and adjustments, a lighter touch is preferable. Using the layer’s Color blend mode also gives you more latitude because its effect is more subtle, which means you can adjust the control points a little more and not be constrained by clipping or brightness changes.