I had this problem too. I had installed Colorful Sidebar and after my Mountain Lion installation, I had no icons at all. (Although when I looked at the individual folders, they still had the custom colored icons.)
To summarize, if you lost your icons from Colorful Sidebar:
- Delete the SIMBL folder. You can go to Finder > Go > Go to folder... and type /Library/Application Support (pro tip: when you type to navigate to a folder like this, or in Terminal, you can type the first couple of letters and press TAB to autocomplete the folder or file name). You should see the SIMBL folder there and can delete it.
- Find the plist files and delete them: Finder > Go > Go to folder... and type ~/Library/Preferences. Delete com.apple.finder.plist and com.apple.sidebarlists.plist
- Restart Finder. (This worked for me; you may have to restart your computer. I also lost my custom sidebar preferences, i.e. the list of shortcuts I had there, so if it's tedious for you to recreate the list you may have to weigh the pros and cons.)
...And agree, I hate the lack of color too. It gives another dimension of visual variance to help select items from the list. (Plus, I have two "Documents" folders there — one my personal docs folder, the other is the Apache webserver Documents folder, and as there's no way to change the sidebar name without changing the underlying folder name, the only way I can tell them apart easily is with the icons.)