The Levels control can be used to neutralize a color cast. Two methods are discussed here. The first method uses the eyedroppers on the Levels dialog box. The second uses the individual color channels on the Levels dialog box. I find each method easier to use if I first create light, dark and mid tone targets in the image. Targeting these areas is discussed on the Finding Lightest and Darkest Areas page.
It is not uncommon for natural light color photographs to have a color cast
to them. This is not always bad. Sunrise/sunset photographs have a
most definite color cast. That is what makes these images so appealing.
However, sometimes a blue/cyan or magenta color cast can be seen in an image.
An example of such an image can be seen in Figure 1. The Levels adjustment
can be used to remove this color cast.
The first method to accomplish this uses the eyedropper tools. But the eyedropper tools are most effective if the image has white and black areas and an area whose color should be (but may not be in the original image) neutral. If not, then we can use the second Levels technique to remove the color cast.
In Figure 2 can be seen the Levels dialog box. In the dialog box are three
eyedroppers. The purpose of the black eyedropper
is to take whatever part of the image it is clicked on and remap it to black.
The purpose of the white eyedropper
is to take whatever part of the image it is clicked on and remap it to white.
The gray eyedropper
behaves differently. It does not remap pixel tones to mid tone.
Instead, it takes whatever part of the image it is clicked on and neutralizes
its color while maintaining the original tone. Neutralizing a color means
making it gray, but not a mid tone gray. I have noticed that, except for
solid colors, the gray eyedropper does not absolutely neutralize a color.
Also, even when used on solid colors, I have noticed there is a slight shift in
tone, but usually ⅓ of a stop or less. However, this does not diminish the
usefulness of the Eyedropper technique.
Using the targeting techniques described on the
Finding Lightest and Darkest Areas page, I created the three Color Sampler
shown in Figure 1. Target #1 is a white area. Target #2, the shaded
side of the pine tree in the middle, is a dark area. Target #3, the pine
tree in the foreground, is a medium toned area whose color we want to be
neutral. We will be using all three eyedroppers to neutralize the color
Here are the steps I used to neutralize the color cast in Figure 1 using the Levels eyedroppers.
The result can be seen in Figure 3. Figure 3 has had no other adjustments
other than the Levels eyedroppers. We can mouse over Figure 3 to see
The pine tree in the foreground now has a nice neutral color. The reds, yellows, whites and greens are also true.
The Color Channels technique is more involved but is used when we need
precise control. Even though Color Sampler targets
were not mandatory when using the eyedropper technique, they are necessary when
using the Color Channel technique.
Once the Color Sampler targets have been created, we will use the information they provide to reduce the color cast. Where is this valuable information? In the Adobe Photoshop Info panel. We can have up to four color sampler targets in a Photoshop document. The image in Figure 1 has three. They are labeled #1, #2 and #3, as shown in Figure 4. When the Levels dialog box is open, the Info panel shows before and after tonal values for each of the targets. The before values are on the left and the after values are listed on the right.
In Figure 4, color sampler #1 has a red value of 201, a green value of 255 and a blue value of 254. Target #1 has a cyan cast to it because green and blue are the dominant color channels and they are of similar amount (green + blue = cyan). Color sampler #2 in Figure 4 has red and green values of 0 and a blue value of 15. Thus, target #2 has a blue tint to it. Target #3, which is the bark of the pine tree, also has a cyan tint to it because of the dominance of the green and blue channels.
When neutralizing a color cast, the intent is to change the RGB after values to be identical to each other. For the white point (target #1), we want to make each of the RGB after values equal to the highest RGB before value, which is 255 in this case. For the black point, we want to make each of the RGB after values equal to the lowest RGB before value, in this case 0. For target #3, we do not necessarily want to make the three channels equal to each other, for this would make the tree bark a true gray. When changing the image for target #3, we want to adjust it enough to leave some color but not make it monochrome.
To remove the color cast using the Channels technique, follow these steps.
When changing the mid tone target, it is important to understand that as we change the red, green and blue channels, we are changing the tone of this area. After we have made our changes, we can use a Curves dialog box to check the mid tone target to see what the new tone is.
The corrected image can be seen in Figure 6. We can mouse over Figure 6 to see corrected/uncorrected versions of the image. Figure 7 shows the Info panel of the corrected image. Target #1 has been neutralized to a true white. Target #2 has been neutralized to a true black. Target #3 has not been neutralized, which is what we wanted. But we eliminated the dominant blue/cyan color cast.
For comparison purposes, lets see how the eyedropper and channels techniques
compare. In Figure 8 are the three Info panels. Figure 8A is the
Info panel for the uncorrected image. Figure 8B is the Info panel for
the image corrected by the Eyedropper technique. Figure 8C is the Info
panel of the image corrected by the Channels technique. (Figure 8C is
the same as Figure 7.)
Evaluating Figure 8A, we can tell the color cast was predominantly in the mid tones since targets #1 and #2 (white and black areas respectfully) do not show much deviation from neutral. Figure 8B shows that the Eyedropper technique did a good job of neutralizing the blue/cyan cast in the mid tones (target #3). It also did a good job of neutralizing the white and black points. The Channels technique, Figure 8C, demonstrates the precise control this technique allows. The whites and blacks were absolutely neutralized and the mid tone area was changed as desired.
The Channels technique can seem overwhelming to use. Therefore, a suggested workflow for removing a color cast is as follows.
The key to using color correction is to be sure to target the correct areas. Do not target deep shadows or specular highlights.
Copyright © 2004 - 2010 Thomas Zuber. All rights reserved. Do not reproduce.