Twenty Eleven Content/widget background color

There’s no way to help with CSS without a link to a live site.

So you have Twenty Eleven Theme Extensions Plugin installed. Check if any color option you want to change might already be there. If not, put in the CSS code under Custom CSS option.

These are for page and footer background.

html #page { background-color: red; }
html #site-generator { background-color: pink; }

For sidebar, it doesn’t work that way. I mean this theme’s markup doesn’t allow for what you are asking for, it wouldn’t look good.