How to install a 4 wire outlet to a 3 wire junction box
In any recently made appliance, green is only legal for one purpose: equipment safety ground.  Green wire cannot be re-tagged as a hot or neutral so it is guaranteed to be ground. 

Bare wire is also only legal for ground.  

They are all grounds.  Simply wire them together. 

Fixtures sometimes have 2 ground wires if there are 2 parts of the fixture which each should be grounded, e.g. a chain-hung chandelier: a chain is not a valid grounding path, so a ground wire must be in the cord running down the chain. The other wire would ground the rose. 

Ground is strictly an emergency current path, and does not flow any current normally.  Electricity needs 2 wires, so 2 more wires must be connected: black and white.  The fixture would work without any grounds at all, but it wouldn't be safe.