The Green Cities Add-on for Cities: Skylines has 6 achievements.
Have all residential zone in your city be specialized Self-Sufficient Housing
Have all office zone in your city be specialized IT-Cluster
Have all commercial zone in your city be specialized Organic and Local Produce
Produce all electricity without using raw materials
Build one of each eco-friendly schools: Community School, Institute of Creative Arts and Modern Technology Institute
Have no polluting industry in your city, only Office Zone and specialized zones