NewBee Here !
When I go to this section of the Atahualpa Theme Options, the ability to change the background color on the center column provides a note about why you can no longer change the PHP because of Wordpress rules. Is there a simple way to change the background color for the CENTER COLUMN, the same way it is done for the sidebars. Thanks in advance.

What version of Atahualpa and WP?
What is the URL?
Do you have all BUGFIX's for your version applied? (they can be found in the 'New Version & Updates' forum and sub forum)
--if not, apply them before we go any further and re-test

You can still style the center column in that area and you can also style the separate Pages and Posts in Style Posts & Pages.