There's a contrast setting the the Universal Access pane of System Preferences however you may be more interested in calibrating the display, which can be done through the Displays pane. 🙂
It's more than 3 years since the question came up and I only discovered it today. Nevertheless, I have the answer and am leaving it here for the other visitors.
To reduce the contrast, it is ctrl-option-command-, (in other words, use the comma instead of the period to reduce the contrast).