It appears that the copper-lookin’ guy on the right was painted using a bronze statue for a model, a practice used by other fresco artists (or indeed, the same guy in multiple houses, probably was well known and in demand).
The Romans really liked Greek sculpture and had plenty of that done, but bronze replicas were preferred for private villas particularly if they were to be displayed outdoors. And like the marble originals, they were painted to look more real (kinda like concrete lawn critters are today).
During the Middle Ages quite a lot of Roman-era marble statuary was basically heisted, dragged off to workshops, and put into a kiln to be rendered into lime for mortar. What an atrocity.
There are theories that the skin tone denoted power and strength vs feminity and “womanhood”.
Many paintings depict dark men and light women.
The rape of ruins and statues, as in your example, was a horrible crime.
Yet maybe some things got preserved that would have been lost. The doors to St. John Lateran are from the Roman Senate [Curia Julia]. And they are awesome.
OTH, the marble from the Colosseum probably would have been better left there.