A colonoscopy in the hands of an experienced team should not be painful at all.  I have been though one myself.  Before the exam starts you will be given some sedating medication through your IV by an anesthesia provider and you will drift off to sleep.  You are awake enough that you are breathing on your own but sleepy enough that you are comfortable.  During the exam you should not feel anything at all.  After the exam you are gassy and bloated for awhile due to the air that is put into your colon during the colonoscopy, but that usually passes rather quickly (and sometimes loudly).  During my colonoscopy I remember laying in the colonoscopy suite talking to my partner who was going to do my exam, I remember some Willie Nelson playing in the back ground then I remember waking up in recovery.  It feels about that quick.