The marble statues and busts seen throughout the movie are white. That's what they are today...but according to many historical sources, 2000 years ago they were painted in various colors.
You just have to categorize this one as part of the director's vision. Not every building in Rome looked like a temple,with lots of majestic columns, and they would've been painted, as well. But Ridley Scott had an idea of what Rome should look like in his movie, and it includes white, unpainted busts.
Sorry to "bust" your bubble here, but you're wrong. The Greeks were the ones who painted the busts various colors, not the Romans. The Romans had white, unpainted busts. So, this isn't a slip-up.
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