1. That's still only one domain. A domain is defined by an installation of Joomla, so you can have as many templates as you need on one domain.
2. A lot of colors can be changed in the template configuration. Anything outside of the configuration you would need to alter the css or images, we do not provide those services.
3. There is a logo tutorial here:http://www.shape5.com/joomla_tutorials.html