I had an issue the other day where 10.4 "lost" all my monitor profiles, and reverted both displays back to Generic RGB. I had to restart to get them back. Take a look at the "color" tab in the displays prefs and make sure you've got good profiles selected. If you've never set up a custom profile for your display, I suggest you hit the "calibrate..." button, turn on "advanced" mode, and go through the assistant.