External monitor has gone negative

My second display monitor- a 20-inch Apple monitor - connected to my Macbook Pro has gone negative. I've tried the control-option-command-8 route, but that only makes the second monitor normal, and turns my Macbook Pro monitor negative.

Rebooting doesn't help, nor does disconnecting and reconnecting the monitor. Can't seem to find an answer on the Apple forums. Any help greatly appreciated.