I need surgery for colon cancer. will i need to wear a bag (colostomy) forever?

Possibly: A permanent colostomy is necessary when a rectal cancer is too close to the anus to be completely removed without affecting the anal sphincter. Occasionally, a temporary colostomy is performed for emergency colon cancer operations (due to obstruction or perforation) or when there is concern about the intestine healing after surgery; these can be "reversed" after a few months.

Depends: Your surgery depends on the location of the cancer. If the tumor is low in the rectum or involves the anal sphincters you may need the area removed resulting in a permanent colostomy (bag). There are options that can decrease the likelihood of a colostomy in this case. This certainly depends on what you have. Ask your doctor. A colon and rectal surgeon specializes in this surgery.

Probably not: Almost all colon cancer surgery is done with a primary anastomosis putting the remaining ends together at the time of surgery. Rarely a temporary colostomy is needed if there is obstruction or other extenuating circumstances. Permanent colostomy is usually only needed for cancers of the very distal rectum.