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Curves: Color cast

or; how to change the white balance

This curves tutorial assumes you have seen the curves (or levels) dialogue before and used it to enhance contrast in your images.
I consider curves the most important tool for anyone working with photographs.

This tutorials explains the process of doing this individually to the color channels (red/green/blue) to change color casts:

The (extreme) example image used here can be downloaded here.

Adjusting your image:

  1. Open your image. As you can see there's only the background layer, and the color is way off:
  2. Add a new curves adjustment layer.
  3. On the top you will find a drop-down list called "Channel:".
    This will allow you to adjust curves individually for each color channel, effectively altering the colors.
  4. After choosing a channel you can use your scroll-wheel while hovering it to quickly flip between the channels to inspect them. In our case it reveals the following three histograms: (image on the right for reference)
  5. Our image was very cyan due to the white balance on the camera being wrong. As you can see the general shape of all three histograms are the same, but the red one lacks contrast (it looks "narrower"). As you can see, the blue channel is the "widest" one. If we adjust the white (and black) points of the channels to clip the "same amount" we will get a neutral white balance:

    Carefully play around with the controls while watching the image - tweak until happy with the color.
    • You may notice that the image actually is overexposed in the blue channel, causing clipping on the flower after adjustment. This is clearly shown if you check the "show clipping" box at the bottom of the dialogue. Unfortunately there's no simple solution to that besides taking better photographs in the first place... ;)
  6. Now adjust the overall contrast using the RGB channel - by adding a very soft s-curve:
  7. Click OK and marvel!
    If you're not 100% satisfied, just double-click the layer to keep tweaking.
    What we did now was manually adjusting levels for the individual channels to neutralise the white balance.

Bonus: If you want to try something more artistic, try editing the curvature between the black and white point for the individual channels to tailor the colors to the feel you want! This photo isn't well suited for this, but here's an example:

That's it. You now know how to use what I consider the most important tool for anyone adjusting photographs.

Masks:

Adjustment layers (like curves) can be used with masks to only affect parts of an image. Masks are a very powerful tool, and would allow you to brighten only the face and darken the sky, while leaving the rest intact. In regards to this tutorial it would be useful if you have a photo where you only need to adjust the color balance on parts of the image (due to mixed light).
A 3-part tutorial on masking can be found on this site.