Welcome to X-Cart! Please start by reading the forum rules, and posting your X-Cart version in your signature (Edit profile, enter version, enter password, save):
http://forum.x-cart.com/viewtopic.php?t=6489
Next, try the manual, found here:
http://www.x-cart.com/xcart_manual/compiled/X-Cart_4xx_manual.pdf
If you still have questions after reading the manual, please search the forum first, as many of the most common questions have been answered multiple times. If you still can't find the answer, post a question. Thanks!
As far as this particular question goes:
Re-install the skin files if you haven't already made major chages to the templates.
All colors are controlled by the skin1.css file.