Why would you have to assume it's arbitrary and political and not just an interesting aesthetic choice?
I wish I could answer that question the way I want to, but I'm already skirting a pretty fine line. The best I can manage is to deflect the question into a related area.
I read alot of science fiction. I mean <i>alot</i> of science fiction. I've read so many books, I've forgotten which ones I've read and find myself checking books out of the library and 5-6 pages into it going, "Wait a minute, I've already read this..."
I really don't care who writes the science fiction. If its a good story, they've got my respect and I'm part of thier fanbase. I've got my signed copy of Samuel R. Delany's 'Babel-17' to prove it.
But I knew Mr. Delany was black (and gay for that matter), long before I ever knew Mr. Delany was black. Why? Because its been my experience reading hundreds if not thousands of books that white writers generally don't have black protagonists. You can be pretty sure that a white writer will have a white protangonist in 90% of his books, and a black writer will have a black protagonist in 90% of thier books. There is too much conscious and unconscious self-identification going on. Unless you put a conscious effort into it, you write what you see in the mirror in the morning. (And frankly, of course they do and what the heck is wrong with it?) Writers seem to have a lot easier time switching gender than switching color. I'm not sure why that is, but personally I would like to believe that unconsciously they realize color is meaningless and adds basically nothing to the story.
I think painting is probably alot like that. Painters paint themselves. They paint who they see in the morning, only in variation. The only way that they are going to honestly paint someone with a different color is if someone close to them is modeling for them and happens to be that different color. Or else, they'll paint someone a different color if something about the scene suggests to them the presence of people of a different color. If I were to see a black character in Rome who wasn't a slave or a gladiator (slave), then I'd assume the blackness came from some conscious decision and feel pretty confident in my pre-judgement. Most of the time, people are drawing from a list of stock characters, regardless of what sort of art they are creating.
Before I'd think otherwise upon seeing a scene of humans of many hues together, I'd have to have some evidence of a well thought out cosmopolitan society that was also multi-hued. It wouldn't be Rome. I'd expect a fantasy city based on Leptis Magna or New Orleans to be multihued. If I saw that, I couldn't really infer anything, except perhaps, that sense it wasn't 'Rome' that first popped into mind someone was making a special effort to break out of the normal sterotype. I'd certainly hope that that wasn't merely to pander to those that reflexively praise racial consciousness as a good thing, but that would be about the limit of that.