City of Chicago Green Alley was uploaded to Flickr

Green alleys incorporate a variety of characteristics:

Permeable pavements (asphalt, concrete or pavers) that allow stormwater to filter through the pavement and drain into the ground, instead of collecting on hard surfaces or draining into the sewer system. The pavement can be used on the full width of an alley, or simply in a center trench.
Open bottom catch basins–installed in alleys to capture water and funnel it into the ground
High-albedo pavement, a lighter-colored surface that reflects sunlight instead of absorbing it, helping reduce the urban heat island effect
Recycled materials, such as concrete aggregate, slag and recycled tire rubber

Other green alley techniques include using proper grading and pitch to facilitate drainage, and using dark sky-compliant light fixtures to reduce light pollution and provide uniform illumination.