I think a post-apocalyptic world doesn't HAVE to look brown and gray, necessarily.  It depends on your variety of apocalypse, but you could have a world that grows back over human civilization.  It could be all green and pasture-y, even Shire-like if you want to go that far.

There was a book you might like:

http://www.worldwithoutus.com/index2.html

For some alternate ideas.  There could still be humans there but maybe so few as to not have a big impact.  That might make an interesting alternative to the Big Pile of Mud.