We stayed in the Yellow Room and it was pretty good but no more. The room was clean but in need of a lick of paint; part of the paintwork on the ceiling was in a bad way due to damp. The shower wouldn't stay in its bracket but other wise was OK and we chose not to mention it anyway. It is in an excellent position, with the Ottaviano Metro station directly outside the door and it's only a 10-minute walk to St. Peter's. The breakfast was acceptable too and there was enough to start you off before a day's walking round the city.

Very convenient location, excellent service, quiet accommodation.
Unusual idea where breakfast is in the room.

My wife and I stayed in the Yellow Room at the Rainbow B&B for one saturday night in june. It was overall a good experience, due to its position in the town and the quality of the room.
Despite the age and aspect of the building, the B&B is nicely refurbished, quiet and spotlessly clean. Our bed was comfortable, the ensuite toilet modern, the room fitted with fridge and m/w oven.
The breakfast was basket style but a large amount of ingredients were provided so the choice and the quantity was good.
The metro station and several bus stops are right in front of the door, while many tourist spots are within walking distance. The Vatican is 10 minutes away on foot.
It is possible to book online and make a downpayment with credit card, however any balance is to be paid in cash. Only caveat: the owner is not regularly at the B&B, so make sure you state your arrival details clearly with her to avoid waiting at the door. She is quite friendly anyway.