So, I've started reading new 52 Batman franchise (Batman, Batman & Robin, Nightwing...) collections and I've noticed the New 52 books all have a colored pattern at the top of the spine. All Batman and Superman books are blue. Flash and Green Arrow books are red, and the various Green Lantern books have a green spine. I've also seen yellow/gold and dark gray variants but can't remember what publications they were on.
I'm kind of curious, do these colors signify some sort of sorting of stories, or were they just chosen for branding and aesthetic appeal?