Apple Aperture 2 - Manual (Page 434)

434 Part III Performing Image Adjustments
Adjusting the Levels of the Red Color Channel
You adjust the levels of the red color channel when you want to remove red and cyan
color casts in the image.
Adjusting the Levels of the Green Color Channel
You adjust the levels of the green color channel when you want to remove green and
magenta color casts in the image.
Red adjustments Effect
Move Black Levels slider right Adds cyan to the shadows, removing red.
Move Gray Levels slider left Adds red to the midtones, removing cyan.
Move Gray Levels slider right Adds cyan to the midtones, removing red.
Move White Levels slider left Adds red to the highlights, removing cyan.
Before Red Levels adjustment After Red Levels adjustment
(moved Gray Levels slider right)
Green adjustments Effect
Move Black Levels slider right Adds magenta to the shadows, removing green.
Move Gray Levels slider left Adds green to the midtones, removing magenta.
Before Green Levels adjustment After Green Levels adjustment
(moved Gray Levels slider right)