How, exactly, does changing a curve change
color? Yep, I’m stuck in the misconception that curves changes contrast.
Open your Curves dialog. At the top, you will see a popup menu that says "CMYK" or "RGB" (depending on which color mode your image is using). Changing the curve at this point will change the overall contrast, or change the overall lightness or darkness, or change lightness, darkness, or contrast in localized tonal areas, depending on what you do.
Now, click on the popup menu. You will have the choice to select each color channel individually. This is how you perform color correction.
By changing the curve in each channel, you can change the color balance of the entire image, or change the color balance of localized tonal areas.
For example, let us suppose you have a CMYK image which was shot under flourescent light. It’s greenish. To change the colorbalance, you would open the Curves dialog, then in the Magenta channel, add magenta to the midtones and shadows. Depending on the nature of the color cast,y ou might also remove Cyan selectively from the hilights and midtones.
If you have an RGB image that is too red in the shadows, you would open the Curves dialog, go to the Red channel, and subtract red from the shadows; it might also be beneficial to boost density in the shadows by adding green and blue.
Curves is your best first line of approach for color correction; it allows you to make both obvious and subtle changes to the color of all or part of your image. It does far, far more than just change contrast.
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