What is an alternative to colostomy bags, referring to colonic cancer?

Depends. If the cancer is low in the rectum or near anus, colostomy has to be done. If cancer is completely blocking, colostomy is also necessary although temporary. Cancers above rectum, usually do not need colostomy.
Not sure of question. If the cancer is in the rectum and surgery will remove it, then a colostomy becomes mandatory. Rectal tumors, may be shrunken first with XRT and chemo treatments and the colon can be brought down through the existing muscles to make a new rectum. If the colon cancer is blocking the bowel, then a bowel prep to clean out can't be done and a colostomy is temporarily needed. Bowel can be reconnected.