When viewing a photo in iPhoto 6, an image may appear darker than expected, or to have the wrong hue, tint, or color balance as compared to another application.
This happens because iPhoto now uses ColorSync to ensure accuracy, and ColorSync relies in part on knowing the source profile. If the photo lacks a color profile, iPhoto may substitute a best-guess profile based on predefined criteria.
For best results, set your camera and (if applicable) adjust the other software in your workflow so that profiles are embedded and/or preserved. Additionally, you should calibrate your display using the Displays preference pane or a third-party hardware calibration device, which is the superior method.