If a sigmoidoscopy scope shown no polyps or cancer in the area tested, does that mean there's no cancer throughout all of the colon?

No: A sigmoidoscopy allows viewing of the rectum, sigmoid colon and occasionally the last part of the descending colon. This is about 25% of the colon and leaves 75% of colon unexamined and at theoretical risk for cancer

Not really: Sigmoidoscopy looks at the sigmoid colon. However there is still that possibility that there may be something in the say cecum for which colonoscopy is a good way to check out that region