There are several ways to do it depending on how you define green. See my File Exchange for three color segmentation tutorials. One that does it in RGB color space, one that does it in HSV color space, and one that does it based on the Delta E color difference metric.
Change the tutorials to get green rather than whatever the color the demo gets. It's just a matter of changing some threshold values. You don't need to change anything with the Delta E method since it does it based on some area that you draw in the image rather than on some absolute color values.
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