Lion and Mountain Lion got rid of the sidebar icons and replaced them with a more sophisticated monochrome version. To easily bring the old icons back you can use SideEffects for Mac.
Although it is great that OS X's design is polished with each new release, some people might be missing the classic sidebar icons introduced in the old days of 10.3 Panther. Initially, bringing the colors back using SideEffects for Mac gave us a strange feeling, but after a couple of moments we got used to the colors and also recognized the convenience of easily detecting folders; when you associate a color with a folder, you will need less time to find the desired one. The shared "PC" icon might be something of an eyesore, but overall, it is an improvement to get the familiar colors back. The program is very simple but it does what it promises, and it does it well. In case you change your mind after trying this software, you can easily get rid of it using the uninstaller.
SideEffects for Mac is perfect for longtime users who miss the old icons and find the new ones too dull. It's simple software you can try out and get rid of easily if it's not to your liking.