- Install SIMBL if you haven't already. Use the 0.9.9 version.
- Download ColorfulSidebar.
- Copy the file ColorfulSidebar.bundle from the Disk Image to /Library/Application Support/SIMBL/Plugins (or ~/Library/Application Support/SIMBL/Plugins).
- Restart Finder.
- Enjoy colorful icons in the sidebar once again!
[crarko adds: I tested this, and it works as described. I do prefer the colorful icons.]