hi,
If the treenodes have the navigateURL, it just like <a> tag at the render time.
so you can try to set the node's color by setting the a's style in css, like below:
<style type="text/css">
#TreeView1 A:link {text-decoration: none; color: red;}
#TreeView1 A:visited {text-decoration: none; color: orange;}
#TreeView1 A:active {text-decoration: none; color: blue;}
#TreeView1 A:hover {text-decoration: underline; }
</style>
<asp:TreeView ID="TreeView1" runat="server" DataSourceID="SiteMapDataSource1">
</asp:TreeView>
Hope it helps.