http://answers.squarespace.com/questions/9355/does-anyone-know-how-to-override-the-text-color-of-one-heading-style
Change font color site-wide for a Heading style:

h2 { color: red !important; }

or for individual pages-

instead of adding the CSS to your custom CSS section, add it to your 'Page Settings' > 'Advanced' > 'Page Header Code Injection' area (of the page you want the CSS to apply to)

But, you have to wrap the CSS in a style tag, like this:  This changes all the Heading 1 text on that page.
 

<style>
h1 { color: red !important; }
</style>