OS X Lion introduced greyscale Finder icons in the side bar of Finder windows. They look nice, but many think their colorful counterparts were nicer. SideEffects lets you replace them.
This simple Mac utility installs components that enable color Finder sidebar icons in both Lion and Mountain Lion. SideEffects is donationware and a beta, so if that makes you hesitant you can skip over it. Otherwise, enjoy the pretty icons!