Its not the 280. I think its either the monitor or a setting in windows. Make sure that windows didnt install any color profiles on your machine. If it did then delete it and the driver for your monitor. It always makes funny colors when monitor drivers are installed with nvidia cards for some reason. Then again, it could just be the monitor going bad.
EDIT: I suppose it could be the 280 if on of the DVI output pins got dislodged or broken or some connection lost. I doubt its the GPU, most likely some pin or connection on the PCB.