3. Dark-colored stools
Another way that colon cancer can affect the bowels is by changing their color. While the color of stool can vary and is largely dependent on the types of food that you eat, generally, it is a light to medium brown. If it appears darker than usual or black, it could be a cause for concern. The tumors growing within a colon that has cancer can become irritated and bleed.
The blood makes its way through the large intestine and mixes with stool, turning it a very dark brown or black. Though not all dark or black colored stool is a cause for concern, as there are several foods and medications that can alter the color of otherwise healthy stool, if you find that your bowel movements have been dark or black for a prolonged period of time (a few days), you should seek medical treatment.