Photoshop Curves
Color Correction

 

The Curves control can also be used to neutralize a color cast.  Two methods are discussed here.  The first method uses the eyedroppers on the Curves dialog box.  The second uses the individual color channels on the Curves dialog box.  I find either one easier to use if I first have created 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

This example uses the same image used in the Levels color correction section.  As can be seen in Figure 1, this image has a blue/cyan cast to it.

 

Eyedropper Technique

In Figure 2 can be seen the Curves 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 and there is usually a slight shift in tone.
 

Using the techniques to identify the lightest and darkest areas of an image (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 is a dark area.  Target #3 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 Curves 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 > Curves.
• In the dialog box, click the white eyedropper .  It is the eyedropper on the right in Figure 2.
• 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 Curves eyedroppers.  Mouse over Figure 3 to see corrected/uncorrected versions.

Like the example in Levels, the pine tree in the foreground has a nice neutral color.  The reds, yellows, whites and greens are also truer.

 

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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.

I find the Color Channel technique with Curves to be slightly more complex than Levels.  However, it has an advantage over Levels.  If we use a mid tone Color Sampler target to help neutralize a color, with Levels we need to get the target as close to mid tone as possible since Levels can only adjust black, white and mid tone.  Since Curves allows us to work with any point on the curve, we don't have to be as precise.  However, for the best results, I still look for a black and white point at a minimum and also add a mid tone point.

Once the Color Sampler targets have been created, we will use the information they provide in the Adobe Photoshop Info palette to reduce the color cast.  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 in Figure 4.  When the Curves dialog box is open, the Info palette 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 don't 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 Curves adjustment layer.  Layer > New Adjustment Layer > Curves.
• Using the Info palette, 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 black point, we want to make each of the RGB after values equal to the lowest RGB before value.  For the white point, we want to make each of the RGB after values equal to the highest RGB before value.
• To correct the black point.  Using the Channel drop down box, 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.  Ctrl + Tab (Command + Tab) until the black point anchor is highlighted.  Click in the Input box.  Using the up arrow key, change the number until the after value in the Info palette matches the lowest before value.  In this example, we want to change the blue channel until its after value in the Info palette equals 0.  Referring to Figure 5, 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.

 

Red channel, Black point Input=0, Output=0

Green channel, Black point Input=0, Output=0

Blue channel, Black point Input=15, Output=0

Figure 5.  Black point adjustments
 

• To correct the white point.  Using the same Curves adjustment layer, select the one or two channels, in turn, that do not have the highest RGB before value.  In this case, it is the red and blue channels.  Use Ctrl + Tab (Command + Tab) to highlight the white anchor.  Click in the Input box of the Curves dialog box.  Using the down arrow key, change the number until the after value in the Info palette matches the highest before value.  In this example, we want to change the value until the red channel 201 in the Info palette 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 6, we note that the red channel white point anchor was changed to 201.  The green channel white point anchor was not changed.  The blue channel white point was changed to 254.

Red channel, White point Input=201, Output=255

Green channel, White point Input=255, Output=255

Blue channel, White point Input=254, Output=255

Figure 6.  White point adjustments
 

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.

• To make the mid tone adjustment, we need to place anchor points on each individual color channel's curve for the mid tone target.  This is easily accomplished by Ctrl + Shift - clicking (Command + Shift - clicking) the mid tone target.  In this case, target #3.  With this one operation, an anchor will be placed on the Red, Green and Blue channels.  An anchor will not be placed on the composite channel.  This ensures the mid tone we are correcting coincides with the target tones shown in the Info palette.
• To make the adjustment, select each color channel in turn and click in the Output box this time.  Using the up/down arrow keys, change the Output number until the desired effect is achieved.  Referring to Figure 7, the red channel was changed to 180.  The green channel was changed to 119.  The blue channel was changed to 114.

Red channel, Mid tone Input=127, Output=180

Green channel, Mid tone Input=127, Output=119

Blue channel, Mid tone Input=127, Output=144

Figure 7.  Mid tone adjustments
 

The corrected image can be seen in Figure 8.  Mouse over Figure 8 to see both the corrected and the uncorrected versions of the image.  Figure 9 shows the Info palette of the corrected image.  Target #1 has been neutralized to a true white.  Target #2 has been neutralized to a true black.  Target #3 was not neutralized.  But that is intentional since our objective was to
remove the dominant blue/cyan color cast in this area, not make it gray.

 

Corrected Uncorrected Figure 8.  Color Cast Removed Using Channels Technique

Figure 9.  Info Palette of Corrected Image

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Comparison

For comparison purposes, lets see how the eyedropper and channels techniques compare.  In Figure 10 are the three Info palettes.  Figure 10A is the Info palette for the uncorrected image.  Figure 10B is the Info palette for the image corrected by the Curves Eyedropper technique.  Figure 10C is the Info palette of the image corrected by the Curves Channels technique.  (Figure 10C is the same as Figure 9.)

Evaluating Figure 10A, 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 10B shows that the Eyedropper technique did a decent 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.  However, the Channels technique, which is represented by Figure 10C, demonstrates the precise control this technique allows.  The whites and blacks were absolutely neutralized and the mid tone area was changed as desired.

Figure 10A.  Uncorrected Image

Figure 10B.  Eyedropper Technique

Figure 10C.  Channels Technique

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Summary

The Color 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.