1. Not allowed here at wordpress.com. Even the VIP sites, which pay thousands of dollars per month, are allowed to remove those.
2. Since your site is set to private, I'm going to have to guess that you mean the nav below the header. The background for that is an image and is set in #subnav.
background: url("images/subnav.png") repeat scroll 0 0 transparent;
This is the URL of the image in case you want to create another to replace it. You will have to upload the image to your media library, get the full URL starting with http and then replace the images/subnav.png between the quote marks.
If you just want to use a solid color, then you can do this and edit the hex color code:
background: none repeat scroll 0 0 #CC0000;
3. If you are talking about static pages, and are talking about the area outside of the content area, then you would have to construct specific rules for each individual page based on the page ID if you want to have each a different color background. This is just an example:
If you want to have the same different color for all static pages, then you would do this: