The *simplest* solution would be to make that sidebar a tiled background, and just apply the background to the content-area. Then, no matter what is longer (the sidebar or the content) the sidebar color will go all the way down.
That's the easiest thing to do.
I have a "sticky footer" tutorial on my site that's old, but it still works (just used it on a site not too long ago), and you can use straight CSS instead of images. The example uses 3 columns, but it's pretty simple to take it down to 2.