Apple Aperture 2 - Manual (Page 426)

426 Part III Performing Image Adjustments
3 Position the target area of the Loupe over a midrange color that is as close as possible
to medium gray, then click.
The tint (hue and saturation) of the gray values in the image is set to remove the color
cast in the midtones.
Correcting the Tint of the Highlights with the White Tint Eyedropper
You use the White Tint eyedropper to remove color casts from the highlights in your images.
Before White Tint adjustment After White Tint adjustment