Color Correction by the Numbers

Often it is more precise to color correct using numerical values rather than visually. This is especially useful when you have something in the image that should be gray or neutral; a color is neutral if the Red, Green, and Blue (RGB) values are all equal. If you have an image that contains something that should be neutral; use this technique. Ideally, you might have something bright that should be neutral (like a cloud, or a white dress) and something middle toned that should be neutral (like concrete or other stone).

Examine the image for a color cast. We can use the color sampler tool to measure the precise values of individual colors in the image.

Select the color sampler tool. It is hidden under the eyedropper tool on the toolbar. It looks like the eyedropper tool with an added target.

We want to sample more than just a single pixel at a time in order to get a more accurate measure of the colors we are sampling. On the options toolbar for the color sampler; change the sample size to "5 by 5 Average".

Select the Info palette, it is likely docked with the Navigator palette.

As you move the color sampler tool over the pixels of your image, the numerical value of the pixels is displayed in the info palette.

Examine the image for a good highlight, and midtone area of the image. For this exercise, these should be areas that you want to be neutral toned. If your image does not have a neutral toned highlight, or midtone area - you cannot color correct for that range within your image; this process is still effective if used for only one range. If you try and make a non-white area white, you can completely throw off the color balance of the image.

We can set fixed points onto the image to show in the info palette. To do this, just move the color sampler over the desired pixels and click on the image. A selection point will be placed on your image and a new point will appear in the info palette.

Select a white point for #1, and a gray point for #2. In this example, I placed point #1 on a white flower, and point #2 on the window curtain.

If we look closely at these numbers, we can see that the white point is not really white; the R, G, & B values are not identical.

Place color sample points on something that should be white and something that should be gray

Create a new Levels Adjustment Layer; Layers>New Adjustment Layer>Levels.

Name it "Color Correction".

Place color sample points on something that should be white and something that should be gray

Correct the values for the white point first. We'll use the white eyedropper tool for this. Examine the values for the white point and pick the R, G, or B value that is highest in this case 249; we will change the value for this point so that the values for R, G & B are all 249, this making the point neutral and near white.

Double-click on the white eyedropper to open the color picker tool; in the R, G, B values, enter the highest value of R, G, or B for

the white point into all of the RGB values -making all of the values the same. Close the color picker tool.

In the levels dialog, select the white eyedropper and click on the white point in the image. This forces the white point to become the same as the value we entered in the color picker tool.

Select the white eyedropper and click on the white sample point to adjust the white value you set in the color picker. The Layers tool adjusts the three channels to make it white.

Lastly, we need to set the neutral point. We can do this using the midtone eyedropper, but this often throws the values of white and black off. So we will make this adjustment manually. For the neutral point, we want to adjust the values of RGB at the neutral point to be the same as the middle value for R, G, or B (this changing the tone of the image least, but obtaining a neutral value). Determine which color needs to be increased (in this case the Red channel) change the channel in the Levels dialog to this channel (Red) and slide the middle input slider to adjust it until it is the same as the neutral value.

Select each channel for which you want to change the midtone value (here change R & B each to 94). Adjust the midtone slider for each channel so the three R, G, & B values match.

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15 Then change the Channel in the Levels dialog to the color that needs to be reduced (in this case the Blue channel). Slide the middle, gray input slider to adjust it until this color is the same as the neutral value.

The midtoned point should now be a neutral color. Press OK to close the Levels dialog.

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