What are the areas marked in green around the taxiways?
It's essentially "fake grass" -- a "no taxi" / "no drive" area, the same as real grass would be.  The concrete may very possibly be stressed to support the full weight of big aircraft, and the areas are painted that way simply as a matter of keeping traffic flow in the designated areas, although it's possible that some areas marked that way may not be in a condition that you'd want to taxi on them.

As far as emergency vehicles go, they have permission to do whatever they need to do when responding to an emergency, which includes driving on anything they need to.  And the people driving the fire trucks will be well aware of any parts of the tarmac that even they need to avoid -- but if it's even close to being able to support an aircraft, it'll support a fire truck just fine.

More than anything else, though, it's a matter of marking the ramp to designate the "don't taxi here" areas from those where you can taxi.  Often, separating a ramp (taxi more or less at will) from a taxiway (ATC clears you where to go, and it's obvious where aircraft will be entering/leaving the taxiway).  (As the overhead image shows, wingtips can &amp; do extend over the green areas; adjacent taxiways separated by such areas would be laid out so that aircraft on respective centerlines would have whatever wingtip clearance is required.)