I'd really like to know ... whether or not the bust is historically accurate.
He is wearing armour and equipment that most would traditionally identify as legionary, yet he is carrying the standard of one of the centuries of the
Cohors V Asturum ("Fifth regiment of foot of Asturians"), a Spanish auxiliary infantry regiment. The standard is identical to the one carried by the
signifer named Pintaius of that unit on his tombstone portrait, now in Bonn.
However, given that the "experts" still have handbag fights over whether or not legionaries and auxiliaries even had uniquely different equipment, I'd say that on certain grounds you could well argue that it's perfectly historically accurate. The reconstruction of the standard of the Fifth Asturians is especially well done.
The usual assertion is that auxiliary
signiferi had the mask of their animal pelts removed, but the representation on Pintaius' portrait is equivocal on that question. There's a triangular area on his forehead that may or may not be the mask of his bearskin. The stone is too decayed to be absolutely sure.
His armour is different from that shown being worn by Pintaius, but you could argue that he's not Pintaius but perhaps a colleague, either of another century of the cohort, or a predecessor or successor. The evidence suggests that standards had a design particular to the unit, so, on that basis, you're restricted to finishing him as a
signifer of the Fifth Asturians, sometime in the 1st or early 2nd century AD.
When it comes to painting, there's some evidence that people of that area of Spain traditionally wore black clothing in ancient times. There's some possibility that this might have been retained as a tradition by Asturian units in the Roman army.