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Bright green color stool could be due to the presence of bile in stool. During a colonoscopy, you were given laxatives that might have disturbed your intestinal flora that is responsible for the yellowcolor of the stool. Also, stool must have passed rapidly through the gut. Other causes could be:
1. Fat rich diet will increase the release of bile pigments and if it moves rapidly, your stool will be green. It occurs during a period of stress or infections.
2. Rarely diet rich in green leafy vegetables can cause green color stools, if malabsorption ensues or due to not chewing properly.
3. If you have any other symptoms like abdominal pain, bloating, nausea, vomiting or green stools, do not stop within the next 12 to 14 hours. Then I would suggest you consult your doctor. You can take probiotics to build up your intestinal flora.