Apple Aperture 2 - Manual (Page 348)

348 Part III Performing Image Adjustments
Choosing a Color Value Option
In Aperture, you can set the Color meter to display RGB, Lab, CMYK, HSB, or HSL
color values.
To choose a color value option for the Color meter:
m In the Adjustments inspector or the Adjustments pane of the Inspector HUD, choose a
color value option from the Adjustment Action pop-up menu.
Choose a color value
option here.