Why oh why is the cover of the last book white??? i have a perfect collection of the entire series on my shelf that i can read over and over and that looks great and now they want to change from black covers to white! Why? i know the prequel is green but that's seperate to the collection, this final, much anticipated, book is!!
I think the cover is sort of an indication of the state of the world at that point. I would assume it's because for the first thirteen the dark has had the upper hand to some extent, but now the light is beginning to unite for the final battle they are prepared to face the dark.
The paperbacks are black, the hardcovers are always different colours (Crossroads was red, Knife of Dreams green, Gathering Storm brown with clouds and Towers of Midnight a sort of emerald colour). It's likely that the paperback of AMoL will also be black, but you'll have to wait a year for it.