Can you, please, detail which exactly color do you mean: the text color? The links color? The background colors of some elements?
Many examples of customizations can be found in
our knowledge base, and this article is the suggested starting point for CSS customization:
http://kb.ecwid.com/CSS-Faq . There are also some ready made CSS themes built into Ecwid, which you can enable at Ecwid control panel > System Settings > Design > CSS Themes. They already contain wide customization over the colors in the shop, so maybe they will inspire you and give an appropriate example.