1. Change in stool consistency or bowel habits: A change in the shape or form of the stool, as well as an increase or decrease in the frequency of bowel movements, may indicate early-stage colon cancer. Blood in the stool can also be a sign of colon cancer.

2. Unexplained weight loss: If an individual experiences weight loss without being on a diet or exercise regimen, especially in middle-aged or older individuals, it could be a precursor to malignant tumors.

3. Rectal bleeding: While rectal bleeding can be associated with various digestive system disorders, the brighter the color of the blood, the lower the origin of the disease. 

Colon cancer involves the development of tumors in the colon, and when passing stool, the stool can come into contact with these tumors, potentially causing bleeding.