The night tours, which run until 11pm, are sold out. Tourists on one visit this week spoke of a "magical" and "romantic" atmosphere in the illuminated monument.
"After sunset is a lot more comfortable," Victor Li, 52, told AFP as he toured the arena and the underground passages where animals and gladiators were once held.
"It's a good idea because it's really hot in the morning and during the day," said Svetlana Spielberg, visiting from the United States, adding that it was "really beautiful" and "more romantic" at night.
The Colosseum was already offering tours two nights a week but doubled that number to four this summer. Officials said some tourists were staying away because of the intensifying heatwaves in the Italian capital.
"The idea of doubling the night tours and doing them also on Fridays and Saturdays was mainly to give access to visitors that prefer cooler hours," said Simone Quilici, director of the Colosseum complex.
The extended opening had been made possible thanks to a grant from the Italian culture ministry and the initiative could be extended into September, he added.
The tours are all guided and numbers have been kept low -- at just 275 people a night. The monument gets up to 20,000 visitors a day at peak times.
The Colosseum, the largest amphitheatre in the world, was built in the first century and was used to host events like animal hunts and gladiatorial games. It could hold around 50,000 spectators.
Marco Pravini, who was visiting from Cremona, said he had also visited the amphitheatre during the day but "at night, the light, the atmosphere is much more magical."
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