Apple Aperture 2 - Manual (Page 349)

Chapter 14 An Overview of Image Adjustments 349
III
Choosing a Color Value Sample Size
You can also set the sample size (pixel area) Aperture uses to determine color values.
To choose a color value sample size for the Color meter:
m In the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose an
appropriate color value sample size from the Adjustment Action pop-up menu.
Choose a color value
sample size here.