Two methods I can think of. One is to use hook_nodeapi and catch the events of getting saved or viewed. Change the body field accordingly. You can also use hook_node_view.
I assume you want to change the bg of body field.
Second easier way is to a use Computed Field. Now add two computed fields named "pre" and "post". Set "pre" to before body and "post" after body in the content "Manage Display"( pl. see the attachment).
In the condtional field "pre" insert the PHP code to take out color field value and print something like: "print ".
In the "post" field close the div tag: "print ""
I suggest you go through any simple Drupal book to get you started with the basics or do a course at Lynda.com.
You can always come back here and ask any question in case you have any query or need help in coding etc.