stomach secretions are fluorescent yellow so if you are experiencing rapid transit times (the need to defecate soon after eating) the yellow gastric juice will often change to green. because you have had these intestinal issues for quite a while it would be prudent for you to see a gastroenterologist so you can determine the underlying reason for your intestinal cramping. this problem can most likely be fixed so don't hesitate to seek help from a specialist.
should i be concerned with green stool. i do get pretty bad cramps in colon and intestin before going and it seems like it happens too soon after i eat and its been present for some time