ok, lets look at this from the begining. you said
I am trying to change the color on a page. The page is div post-11, so says Firebug.
when you refer to PAGE that is the entire page but then you say the page is div post-11.
the div with a class of post-11 is not the page but a small part of the page, the post. this would be a small part of the page and the CSS insert you said you used has a totaly invalid CSS Selector. If the CSS selector is invalid then the rule will never be applied.
Larry then gave you the css to change the background for a post with a class of post-11, assuming from our example, that you wanted to change the post background (see not having teh URL to look at what you want to change, we have to make some assumptions)
I suggested adding an !important because of the CSS selector precedence rules. It would caue this rule to be applied in case another rule was also being applied.
Then you stated you are trying to change the background color of the page, which is why I told you to use the body.page-id-11.... css - something that I tested and it works.
another issue could be that you are placing it at the end of your CSS inserts but you have a syntax error earlier which is causing all teh following css to be ignored.
so remove all your css inserts except for my last suggestion and see what happens. But remember that we can only go by the information you give us, when there is misleading information, or a answer is given using the wrong termonology the results might not be what you expect.
But keep working on it, if you can't show us the page, then attach a export of the settings ech time so we can look at whet you are actually doing.