Both the Levels and Curves dialog boxes make it easy to edit by
color channel. But is it possible to use Levels and Curves to edit by
color ranges? Yes, but not directly. Basically, we create a
selection of one or more color ranges and then use this selection in a Levels or
Curves adjustment layer mask.
An example is shown in Figure 1. Figure 1A is our image. By clicking
Select > Color Range, we get the Color Range dialog box as seen in Figure 1B.
By using the left most eyedropper
and clicking the blue sky, the Color Range dialog box created a selection for
us. The Color Range dialog box shows us the selection as a mask.
White areas are selected and black areas are not selected. We can use the
Fuzziness slider to increase or decrease the selected color range. After
making the selection, click OK and the classic marching ants will appear on the
image. We now have a selection. Next, click Layer > New Adjustment
Layer > Levels, a Levels adjustment layer is created for us. This can be
seen in Figure 2. Looking closely at Figure 2A, we can see that the Levels
adjustment layer mask is filled. Figure 2B shows the contents of the layer
mask. It is identical to the mask created by the Color Range command.
Because a selection was active when the Levels adjustment layer was created,
Photoshop automatically filled in the layer mask based on the selection.
The end result is whatever adjustment we make with the Levels command, only the
area of the image containing the selected color range will be affected.
Figure 1A. Flowering Trees and Shrubs
Figure 1B. Color Range Dialog Box
Figure 2A. Layers Panel with Layer Mask
Figure 2B. Levels Layer Mask Enlarged
When using Levels or Curves to adjust tone, I usually
change blending mode to Luminosity. If left to Normal, we run the risk
of adjusting color as well as tone.