Using a reflective sensor to detect green-
If you had checked the data sheet of the part you listed, you would have noticed that the CNY70 is an infrared emitter/detector. The detector probably doesn't care, but it is behind a filter that only lets infrared reach the detector.  The emitter only sends infrared.

No filter you could possibly use will make it respond reliably to green objects because green is just too far away from the operating spectrum of the part. 

To do it using reflection, you will need a green emitter and a detector with a green filter.

Phototransistors have a very wide spectrum response, so no problem there.
You need to pair a green LED with a phototransistor, and put both of them behind a piece of green glass or plastic.

As @BrianDrummond noted in a comment, a detector with a green filter will detect green objects, but also white objects because white also reflects green.

So, you can only use a green detector if you can guarantee that all objects to be detected will only have colors that contain green, and that all objects that should not be detected will not reflect green.

Alternatively, you must detect all colors separately (RGB) and evaluate the three detectors to decide what green is really green.