You don't need tons of time at the Colosseum anyway unless you need to have the deeper details. For the photo and go crowd the typical tour will be fine of about 60 minutes. There are additional tours they offer that allows you to go down into the pits and "underneath" the floor of the arena.
Disney is in port for 12 hours. Its 90 minutes each way from the port to Rome, leaving you with 9 hours of touring. 3 hours at the vatican museums, maybe an hour at the basilica (which is a bit of a walk from the museum exit). Then 90 minutes for lunch. A photo and go at the Trevi, and travel time I assume by bus or van between all the places. You should have probably about 90 minutes at the Colosseum and the immediate area around it, including the Constantine Arch, and viewing from a short distance the hill, and the old city.
Not a bad day for a quick visit to Rome. I was there for 4 days and felt like I barely got to take it in.