Is a pet scan used for colon cancer detection instead of a colonoscopy?

No. Pet scans are used for staging cancer (assess if it has spread to other tissues), not for screening.
No. This is not a primary modality to identify a cancer, especially a colon cancer. It is used to better define metastatic disease in appropriate situations. A colonoscopy is your best test to identify a colon cancer if present.
No. The standard method to detect colon cancer is a colonscopy- plus it gives the doctor a chance to get a biopsy at the same time. A pet is a very useful test for figuring out the extent of colon cancer once a diagnosis has been made- it is not used as a test to detect cancer at present..