Exporting figure as pdf using savefig() messes up axis background in OS X

I'm trying to change the axis background on a plot where several imshow() calls render images in various locations via the extent parameter.

When I save a pdf of the figure using savefig(), I lose the background color if the axis displays more than one image. Note that this doesn't happen when exporting a png of the same figure.

[Example code and the corresponding output](http://stackoverflow.com/questions/15885763/exporting-figure-as-pdf-using-savefig-messes-up-axis-background-in-matplotlib) [stackoverflow.com].

Could this be a bug with the MacOSX backend I'm using?

__Note:__ I'm using matplotlib 1.2.1 installed via `pip` running on Python 2.7.4