Are colon polyps cancerous?

Some. There are different types of colon polyps. Some will not turn into cancer. But many (adenomatous polyps in particular) have a high likelihood of turning into cancer if not removed. We think that most colon cancers start as polyps. That is why we recommend colonoscopy at age 50. If you have a precancerous polyp, it can be removed before you get cancer.
Certain ones are. There are different types of colon polyps. Tubular adenomas, tubulovillous adenomas, villous adenomas, serrated adenomas and sessile serrated adenomas can progress to malignancy. Hyperplastic polyps, inflammatory polyps, mucosal prolapse polyps and hamartomatous polyps do not have any malignant potential. Villous polyps and adenomas over 1 cm have highest chance of progression to malignancy.
Maybe. Colon polyps may, on occasion, contain cancers. That is why we are screened by colonoscopy, and polyps are removed during colonoscopy.