Rome streets and sidewalks can be notoriously difficult to navigate as a pedestrian, let alone in a wheelchair or with a walking stick. However, there are plenty of options for people with limited mobility to visit Rome's attractions.
The Colosseum is certainly visitable in a wheelchair. There is a ramp for entry, and you can move around parts of the lower level. There is an elevator to reach the second level.
You can also visit Saint Peter's basilica in a wheelchair. There is a special ramp for wheelchairs. Visiting the Vatican Museums in a wheelchair is very doable. You may miss parts of the normal itinerary but of course you will see the Sistine Chapel.