I researched this pretty extensivley about a month ago and here is what I found out re: CT scans and colon cancer.
First of all, there are several ways a CT scan can be ordered, without contrast, with IV contrast and with oral contrast. IV contrast helps the vessel-rich liver show up, to look for mets. This is primarily what a post-cancer CT scan is for, so I am sure your health care provider has at least ordered your scan with IV contrast. A scan without contrast compared to the scan with IV contrast is also useful in distinguishing mets from other lesions on the liver, mainly cysts. I can't recall the specifics, but one of the two looks different with the IV contrast, and the other looks the same both with and without. The radiologist who reads the scans knows. I suspect your doc. will, at minimum, order a CT scan with IV contrast and a CT scan without contrast. Portions of the colon can show up with IV contrast, but not to the extent that it is considered diagnostic for conditions of the colon.
The oral contrast is what helps the colon show up. This *can* show some masses and tumors, esp. if they are involved with the muscle of the colon wall. Because your CT scan is primarily being used to look for mets, it may well be that your provider will not order oral contrast. Rather then pay for an extra scan with oral contrast, you might consider moving up your scope. It is the definitive diagnostic tool, and CT scans can miss smaller masses and precancerous polyps.
Also remember that this time of year, there are lots of GI bugs going around. Given your worry and your history of colon cancer, it might well be worth it to move up the scope and put your mind at ease.