If you're like me and like to read your feeds sorted by tags, you'll find yourself jumping from blog to blog. Some Greasemonkey styles for Google Reader drop the feed name to save space, making it hard to tell articles apart.
Color is an efficient way to cue people that things are changing. Unless you're color-blind, I guess. But since I'm not, I appreciate the Google Reader - Colorful List View script because it makes it simple to tell that there's different feeds.
Note: updated to show correct script