Sure, this tweak won't make your system any more usable, but it'll certainly make it a little less depressing. All you need to do is download and install SIMBL (if you don't have it already), then download and drag the .bundle file in this DMG file to:
If the folder doesn't exist, you can create it. Then, just open up a Terminal window and restart the Finder with:
And voilà! You should see the Finder's sidebar has returned to its old, colourful self. Hit the link for more useful Finder tweaks in Lion.