Photoshop Levels
Color Correction

 

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.

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Neutralizing A Color Cast

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.
 

Eyedropper Technique

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 targets 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 cast.

Here are the steps I used to neutralize the color cast in Figure 1 using the Levels eyedroppers.

• Make sure the caps lock key is on.  This will change the eyedropper cursor to a crosshair .
• Create a new adjustment layer.  Layer > New Adjustment Layer > Levels.
• In the dialog box, click the white eyedropper .  It is the eyedropper on the right.
• Place the eyedropper cursor over the image.  When we do this, the cursor will become a crosshair.  This is the same crosshair used by the Color Sampler targets.
• Position the eyedropper crosshair over the white target, in this case #1.  When the two crosshairs are perfectly aligned, they will merge and become one.
• Click once using the left mouse button.  Do not hold down the Shift key while clicking.
• Repeat for the black eyedropper using the black target, in this case #2.
• Repeat for the gray eyedropper using the mid tone target, in this case #3.

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 corrected/uncorrected versions.

The pine tree in the foreground now has a nice neutral color.  The reds, yellows, whites and greens are also true.

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Color Channels Technique

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.

• Create a Levels adjustment layer.  Layer > New Adjustment Layer > Levels.
• Using the Info panel, note the RGB values of the color sampler targets.  They are in the format before/after.  The intent is to change the RGB after values to be identical to each other for the black and white targets.  For the white point, we want to make each of the RGB after values equal to the highest RGB before value.  For the black point, we want to make each of the RGB after values equal to the lowest RGB before value.
• To correct the white point.  Using the Channel drop down box, select the one or two channels, in turn, that do not have the highest RGB before value.  In this case, they are the red and blue channels (Figures 5A and 5C).  Click in the right (white) Input Levels box in the Levels dialog box.  Using the down arrow key, change the number until the after value in the Info panel matches the highest before value.  In this example, we want to change the value until the red channel 201 in the Info panel equals 255.  When we edit the blue channel we want to change the blue channel white point value until the blue channel 254 equals 255.  Looking at the three dialog boxes in Figure 5, we note that the red channel white point was changed to 195.  The green channel white point was not changed.  The blue channel white point was changed to 254.
• To correct the black point.  Using the same Levels adjustment layer, select the one or two channels, in turn, that do not have the lowest RGB before value.  In this case, it is the blue channel.  Click in the left (black) Input Levels box on the Levels dialog box.  Using the up arrow key, change the number until the after value in the Info panel matches the lowest before value.  In this example, we want to change the blue channel until its after value in the Info panel equals 0.  Referring to Figure 5 again, we note that the red channel black point was not changed.  The green channel black point was not changed either.  The blue channel black point was changed to 15.
• If further correction is still needed, then correct the mid point.  This change is more subjective than numerically precise.  In this case we know that the tree bark has a blue/cyan cast.  To undo this dominance, we want to change the values such that neither blue nor green + blue is dominant since green + blue makes cyan.  Therefore, I changed the values so that red and red + green are dominant by changing the gamma value for each color channel.  Using the Channel drop down box, I selected each of the channels, in turn.  I clicked in the middle Input Levels box.  Using the up/down arrow keys, I changed the numbers until I got the desired effect.  Referring to Figure 5 once more, the red channel gamma point was changed to 1.23.  The green channel gamma point was changed to 0.91.  The blue channel gamma point was changed to 0.95.
  
   

 

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.

Corrected Uncorrected Figure 6.  Color cast removed with channels technique.  Mouse over to see uncorrected version.

Figure 7.  Info panel of corrected image

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Comparison

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.

Figure 8A.  Uncorrected Image

Figure 8B.  Eyedropper Technique

Figure 8C.  Channels Technique

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Summary

The Channels technique can seem overwhelming to use.  Therefore, a suggested workflow for removing a color cast is as follows.

1. First, locate white and black areas in the image that should be corrected and use the white and black eyedroppers and click these areas. If the results are unsatisfactory,...
2. then locate a color in the image that should be neutral and is as close to mid tone as possible.  Use only the gray eyedropper and click this area.   If the results are still not satisfactory,...
3. then use the Channels technique.