Correct as to how to change the colors. If you want to do a mix-and-match of colors or images, you can create classes for books and topics in the skin, specify different images to use for that book or topic, then in the TOC Editor, in the Properties screen for a TOC entry, call the class you created. For instance, you could use green books for "basic" information and maroon books for "advanced" features/info.
Lisa Eagles may soar, but weasels aren't sucked into jet engines. Warning! Loose nut behind the keyboard.