Simple way to apply colour schemes across your site

You could use themes, but an easier way is to create separate stylesheets that override base styles that you have created (with different colour schemes). Then within each top level site that you want to apply the colour scheme to, navigate to Site Settings | Master page, and in the alternate CSS URL section, select the 'Specify a CSS file to be used by this publishing site and all sites that inherit from it' radio button and then specify the path to your alternate CSS file. Pretty straight forward really. :-)