Please use Pubmed.gov to look for peer-reviewed journal articles pertaining to colon cancer and color-blindness. I’m interested to know if there is a relationship between a person being unable to see the color red and a late-stage diagnosis of colon cancer. One of the early warning signs of colon cancer that should prompt a person to go see a doctor is blood in their stool after using the toilet. If a person is unable to see the color red, they may miss this early warning sign. That person might not go see a doctor until cancer has spread and begins causing pain or there is some other indication of a problem.