I guess you can find a lot of nice ways to shade, color, hatch an area in the "Amazing Images" E-Book.
Byrge Birkeland also had posted a sheet (unfortunately Prime) specifically about shading a region here:
http://communities.ptc.com/docs/DOC-3495
Some other link which might be of interest:
http://communities.ptc.com/message/205924#205924
http://communities.ptc.com/message/158074#158074
http://communities.ptc.com/message/233004#233004
In your case you might first define a piecewise function following your horizontal marker and then the green line and use bars or error bars to shade the area underneath.