I have found the ATAC website to be incomprehensible so I am going to the true source of all information: our travelers' helpline. Staying near Campo Fiore and looking for bus options to/from the Coloseum. Any advice? Thank you.
Walk to Corso Vittorio Emanuele near the corner with via di Torre Argentina, then take the #87 or the #571. Get off at the Colosseo.
Take the 87 or 571 from Largo del Torre Argentina, a major transit hub. Read Roninrome.com for more info on the buses. http://www.roninrome.com/transportation/bus-metro-tram-rome-italy