Be sure to check off as you go along.
Create a Levels adjustment layer, or reopen its window by double clicking the Levels window thumbnail in the Levels adjustment layer.
Go to the Channel box on the Levels adjustment window, and change RGB to red, green, or blue.

Adjust the color for the chosen channel using the gray slider (in the middle).
Or, you can change 1.0, in the middle Input Levels box, to another number.
When you change one color, its complimentary color also changes in the opposite way
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