Aperture 1.1 or later includes a built-in color meter that can display RGB, CMYK, and lab values.
Aperture 1.0 does not include a pixel color meter, or "dropper," so you can use this workaround: Look in your Utilities folder (located inside the Applications folder) for DigitalColor Meter, which will provide the desired function. Be sure to familiarize yourself with the Preferences in DigitalColor meter. The option to "Float window above other apps" may be particularly convenient while color balancing in Aperture.