Tutorial: Correcting Color Casts

Color Cast Correction

This tutorial will help you to understand and to fix color casts in digital images. Color casts occur in both scanned images (from prints, slides and negatives) and in digital images from a digital camera.

The example image we will be working with has a blue cast because it was shot on a sunny day, with the blue of the sky casting a blue glow. In many cases, pictures taken outside will have at least a slight blue cast. Click on the image to download a full sized copy to use to try the techniques described below.

Open your image in Photoshop. If you are working with your own image, please make sure that you are working with a copy so that you can always revert back to the original if you don't like the results.

Open the "Levels" tool (Image->Adjustments->Levels or use Crtl+L).

First, let's see how well the automatic correction in Photoshop works. Click the "Auto" button.

You should see some changes in the color in the photo. when you click "Auto", Photoshop adjusts the black and white points for each individual color channel (Red, Green and Blue) so that the histogram for the channel just touches the black and white points of the curve. Sometimes this works great in reducing or eliminating a color cast, but often it doesn't succeed in completely removing the color cast from an image. In the case of the example image, it does reasonably well, but it is not perfect. More commonly, the image will still be off in significant ways. For this reason you should understand a bit more about doing a manual color correction.

Next, let's do some manual correction. Undo the effect of the "Auto" adjustment by holding down the Alt key and clicking on the "Reset" button (was the "Cancel" button). Go to the RGB display by typing Ctrl-` (use Crtl-1 for Red, Ctrl-2 for Green, Ctrl-3 for Blue). Slide the sliders just a little into the tail (the tail is the part of the histogram the slopes down into the bottom) and move the slider just a little into the tail. You can hold "Alt" while sliding to see what details you are forcing to black or white. At this point don't worry if the image is a weird color. Moving the black and white points for the RGB graph should improve the contrast of the image - giving more blacks and whites. It won't however change the color makeup of the image. The image below shows this type of adjustment for the Blue channel.

Black (left) and white (right) point sliders	Histogram tails	Adjusting both white and black points for the blue channel

In some cases adjustment of the black and/or white point sliders for an individual color channel or two will correct the color cast. However, for some images some additional work is required.

One way to correct the color cast is to use the gray dropper in the Levels dialog to show Photoshop a spot in the image that you believe should be gray - and presumably is not currently showing as gray. If your picture has gray spots or gray shadows in it select the gray eyedropper (the middle one) and click around in the gray spot until you get the desired correction.

In this example picture, there is a shadow on the brim of the flower that we used to set the gray point. When you click with the gray dropper, Photoshop adjusts the middle sliders in each color channel to make the spot that you clicked "gray". All the other pixels are moved accordingly.

If your picture does not have gray spots this might be a little harder. First your need to figure out what color cast your picture has, then you need to think of the opposite of that color and add the corresponding color to cancel out, this can take some guess work and experimentation. If you have no clue you should mess around with the middle sliders on each color channel to see the effect, and doing a little adjustment at a time, until the picture is what you want.

Here are the before and after pictures:

Before Color Cast Correction (click image for a full-sized version in a new window)	After Correction (click image for a full-sized version in a new window)