rainbow-blocks-bg is an Emacs mode that highlights the background of code blocks delimited by parenthesis, brackets, or braces. The highlight color is determined by the blocks nesting level.
This package is a fork of the rainbow-blocks.el package and only applies minor patches such that the background is highlighted rather than the foreground text.
This was just a quick hack. I'm not sure if this will be that useful but I thought it was cool.
I'm not sure if I'll add this to MELPA but for now download the rainbow-blocks-bg.el file into your .emacs.d directory or wherever you keep external libraries. Then add this to your init file.
(load-file "path/to/rainbow-blocks-bg.el")
(global-rainbow-blocks-bg-mode)