"It was believed the paint would not have survived at all," Rea said.
Deriving from iron oxide and clay minerals, the red color could be used without any binding material.
"As the color could withstand two-three years, it had to be periodically applied on the carved numbers," restorer Cinzia Conti, said.
Gladiator Chews Out Ref From Grave
Commissioned by the Emperor Vespasian in A.D. 72, the Colosseum was opened by his son Titus in A.D. 80 with 100 days of gladiatorial bouts, beast battles and public executions.
It was the largest building in Rome, covering a little over half an acre and standing 164 feet high against the Roman skyline.
It continued to provide the ancient Romans "panem et circenses" - bread and circuses - until shows were banned in the fifth century, about 40 years before the fall of the Roman Empire.
Rea's team expects to find other traces of colored numbers inside the monument as the restoration proceeds.
Image: Traces of red color in the Roman number X (10). Credit: Soprintendenza Speciale per i Beni Archeologici di Roma.