Someone has discovered a way to alter the image files that control the appearance of the iTunes 5 user interface. You can read the article here.
If you wanna change the color or try to mod it further you can by finding the iTunes.rsrc file inside the iTunes.app. This file should be in the base resource folder in iTunes. Anyway open the file with an application like Resorcerer and when the file is open under the Types column go to PICT which of course is the picture resources. When selected slide down to resource ID 1025 and from there its pretty easy to see which files are associated with the UI in iTunes. I used Photoshop and the Hue adjuster to make my ID 1025 and ID 1026 green and the ID 1027 and ID 1028 blue.