Stop worrying. Stool color reflects what you've been eating: fatty or not fatty, high or low fiber, plant or animal, etc. Etc. The two colors that worry me most are black (bleeding in the stomach) or red (bleeding in the intestines). If you feel good and there have been no changes in your health, relax. If you don't feel good and your health has changed, then stool color might be a clue. See your doctor.
Blocked bile flow. If the grey colored stools are associated with yellowing of the eyes or skin, abdominal pain, or vomiting, this should be evaluated promptly for possible blockage of the bile ducts.
Grey stool. Due to insufficient bile. May signify a condition (biliary obstruction) where the flow of bile to the intestine is obstructed from a tumor or stone. Please see your doctor asap to run tests and make a diagnosis.