I'm not a Denver resident, though we own a house there but I think this is a good idea. 

Only 20% of a roof would have to be green in some manner, and solar panels are easy enough now that it should be a simple addition to a new or rebuilt commercial roof. That plus some recycling initiatives would give the buildings an easy way to advertise their green status, plus they would probably get some tax break or write off on the effort. If they go with solar panels, they can use the electricity and reduce their cost to operate down the line. 

Compared to the cost of building a new building, adding rooftop solar or garden space would be a minor addition and would provide some benefit. I bet a roof with a garden has less cooling costs too.