Color Cast Correction
Color Sample Point.
The neutral sample points used to set the individual channel midpoint tones are shown in the image below. I intentionally introduced a color cast into this sample image to better demonstrate how curves can improve color cast.
Use Color Samplers on neutral areas. It is important to find a sample point that is close to neutral and as close to the midpoint—128—as possible, since you will adjust the midpoint in one or two of the individual color channels: Red, Green, and Blue.
Red and Blue channel Color Cast adjustments.
Uncorrected color cast (left image) and corrected color cast (right image).
This Color Cast correction removed Red (by increasing the Red channel’s midpoint density value) and removed Yellow (by increasing the Blue channel’s midpoint density value). The area sampled by Color Sampler 2 was more neutral than that of Color Sampler 1. Remember that 0 is full black and 256 is full white. To increase density, you lower the RGB numbers. In some cases, you won’t have a truly neutral area. To accommodate this, I set several color samplers in areas closest to neutral and averaged the readings as a starting point. Selecting an appropriate neutral midtone value is the most important part of this process. It will take a bit of practice.
In most cases, you will need to adjust two of the three color channels. You generally move the two outlying numbers toward the middle value of the RGB numbers. For example, if your RGB numbers are R 145, B 139, and G 150, you would move the Blue channel midpoint until the Color Sampler in the Info palette reads 145. Then select the Green channel and move the midpoint until it reads 145 on the Color Sampler in the Info palette. Sometimes (when two of the three RGB numbers are aligned—for example, R 120, B 114, G 120), you will only need to adjust one channel. In this example, you would move the Blue channel up to 120.
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