First step is to minimize everything so you can see your desktop. Right click the desktop and select properties. A menu should appear. Go to the settings tab. You shouls see several dropdown menus/slider bars labeled things like 'Screen Resolution' and 'Color Quality.' In the 'Color Quality' one, select the higest number of bits available (8, 16, 32, 64, etc.) Once you have selected the highest available setting, click the 'Apply' button and/or 'Ok' afterwords. You may be prompted to restart your computer. If so, restart it. This should fix your problem, if not, ask me again.
NOTE: 256 Colors is not a high bit setting, it is a very low one. That being the case, don't select that option!