Community titles on posts within the stream and notifications have been changed from black to green, matching the shade used on the Google+ Communities logo. Note that this update has only rolled out to the desktop web, and not mobile as of posting.
The move was most likely made to easily identify where posts have been shared to at a glance (without, you know, reading). Colour coding posts is a great idea, though posts shared to communities are the only ones that are using this system right now. It does however, make you wonder if there's going to be some major changes to sharing on Google+ in the future...
Thanks +Mohamed Magdy