Can kub show polyps and/or colon cancer?

Very unlikely: Detail not good enough for polyps, but if a colon cancer has obstructing signs, it may give clues on KUB, suggesting other more detailed studies, i.e. CT.

Possibly: It's not unheard of to see abnormalities on a KUB but that's not the correct way to look for polyps. A colonoscopy is a very reliable way to detect polyps.

Not usually: Polyps and Colin cancer are soft tissue conditions, and do do not typically show up on X-ray. The main exceptions to this are if they are calcified, if they are causing a bowel obstruction ( you can diagnose the obstruction but not the cancer), or if contrast is given in the GI tract first ( known as a upper or lower GI series). Ct scan is the radiologic test of choice, colonoscopy shows as wel