What are the common symptoms of colon cancer?

Sometimes: They don't have any, and discovered on screening colonoscopy. These have the best prognosis. The earliest symptom is probably bleeding, but at first you need occult blood testing to detect small amount of bleeding. Then gross blood, then obstruction and perforation symptoms, such as cramps, pain, fever, chills, and even peritonits. These have much worse prognosis, so get colonoscopy at age 50.