Try right clicking on an empty part of your desktop and click on Properties. Click on the Themes drop down menu and find Windows XP theme. Select it and click OK or Apply. That should restore everything back to the original Windows XP color scheme.
If you only want to change the color of certain items, go to that same Themes page but this time click on the Appearances tab. Next click on the Advanced button to change the color of any individual item. You can save the new color scheme by naming it and saving it.