You should try to reinstall the video drivers. Visit the manufacturer site. If you
need help to locate the site, download Everest
Home Edition, run it, and read the information on your video hardware. At
the bottom of the information should be a link to the manufacturer's drivers download
page.
Make sure you install the correct drivers or you will have worse problems than
an incorrect color mode. Incorrect drivers can harm your Windows installation, and
the only way to recover it is booting in safe mode to uninstall the incorrect drivers.