The Preserve Color option helps you eliminate color distortion inherent to popular image adjustment algorithms such as Brightness / Contrast, Levels, and Tone Curves:
The algorithm used for the Preserve Color option is based on the HSY Color Model which separates the image into hue, saturation and lightness channels taking into account the human perception of color. When Preserve Color is turned on, only the lightness channel of the source image is affected by the adjustment, the hue and saturation are left unchanged.
Technically, the following steps are performed. First, the original hue and saturation are extracted from the unmodified image. Then the required adjustment (Brightness/Contrast, Levels etc) is performed, and the lightness channel is extracted from the modified image. Finally, the result is assembled from the modified lightness channel and the original hue and saturation channels.
