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).