Mix of dark and light green stool especially after eating certain fruits i.e. grapes n dull stomach ache for two months. possible causes?

Changed stool color: The color of stool varies with foods, speed of emptying of the GI tract, medications & supplements ingested, gut flora (unique for each individual), additives (like blood), subtractions (like absent bile), infection, inflammation, & other factors. Consider what you ate in the last few days, but if greenish color is unusual for you & persists or is accompanied by other symptoms, see your doctor.