One of the changes Apple made to iTunes that several people have been disappointed with is the app's lack of colors in the sidebar, making the window look out of focus and a bit bland. Some people have looked into hidden settings and other ways to modify iTunes to bring the colors back.
Unfortunately, changing the sidebar contents is not as simple as entering a command in the Terminal. Instead, you must alter the ".rsrc" file in the program package, which is not a very straightforward task.
To install the resource files, first download them and then unzip or unpack them so that the rsrc file is available. Then right-click iTunes and choose "show package contents" from the contextual menu.
Locate the "iTunes.rsrc" file in the /Contents/Resources/ folder, and back it up by copying it to the Desktop or another safe location. Then copy the modified iTunes.rsrc file into the /Contents/Resources/ folder, replacing the one that you just moved.
After this is done, launch iTunes and you should be good to go. You may want to run a permissions repair on your hard drive after doing this, but as long as the file is readable by your account and the system it should work.
Keep in mind that when doing this you are tampering with iTunes' internal components, and there is no guarantee that it will work properly or work without any odd side-effects. Nevertheless, the edits being done are only for visual elements and not for any binary files, so it is highly unlikely this will result in problems. In my testing, the program runs just fine, and I have some color back in the sidebar.
If you decide you want to undo this change, either copy the backed-up rsrc file into the iTunes package or remove the iTunes application and reinstall it from the iTunes Web site.