If you'd like to change the default color scheme on your site, you can actually do that quite easily within the builder. Each template has several different color schemes to choose from.

Take a look at the colors you have to choose from by logging into the builder and accessing "step 2 - Design".

If you look to the upper right of the screen, you'll see which colors you can choose from. Clicking on each thumbnail will change the template preview image on the left of the page. If you want to see what your site will look like with a certain color scheme, click "Preview" (in the bottom right). If you like what you see, save the color scheme to your site by clicking "Save Changes".

Typically, the next step is to change or modify a site banner.  Please see How Can I change the Banner? for further information.  For an overview of all the classes, go to 102: Customizing your Site's Look and Feel.