how to make sidebars length of page

im trying to make my sidebars the full length of the page.
I have put a background colur to my sidebar, the background shows up in the sidebar behind the blocks, but when there are no more blocks the backgrounds no longer there. I would like the sidebar to be the full length of the page, not just the height of the blocks. I cant give it a fixed height, as the content varies per page.

Is there something I can put to make the sidebars extend to the bottom of the page, or is there anything I may have in my theme thats stopping this?