Green stools, little black dots in stool and also when i wipe. scared this may be rectal/colon cancer at 18. what are the chances?

Very low: Colorectal cancer does not cause this specific character of stool, and while not impossible, the chances are close to zero you have colorectal cancer. Best to see your primary care provider for a more complete evaluation to be sure. Hope this helps!

Unlikely to be signi: at your age and this is not likely to be colon cancer. It is likely some thing related to either your diet and/or some inflammatory disease of the bowel. Go see a gastroenterologist