It was still fashionable, in the seventeenth century, to impute the distinguishing shades of human character to the influence of complexion.
First off, the rhyme is entirely independent of the discussion about ‘complexion’ (which, of course, he and Dryden were using in Galen’s sense of ‘mixture of the four humours’, not skin colour) that that terrible old faker Scott chose to drag it into.
people did get around much more than one would think, so an African in the Scottish border country of the seventeenth century, while highly unusual, wouldn’t have been unheard of.
But in practical terms pretty much unheard of, except in the environs of a ‘great house’ that had black servants – and not only are ‘great houses’ very thin on the ground in the Borders, but the slave trade was only just starting to gather momentum. Not anywhere near enough of a likelihood that it was worth devising proverbs advising how to deal with the eventuality. (FWIW, I was born in 1956 and I can still well remember when even in villages less than 50 miles from London the sight of a Real Live Black Person would have the local people gawking, even though they had all seen black people on television.) In any case, the rhyme is clearly about hair colour, not skin colour, unless we are going to argue that ‘brown man’ and ‘red man’ refer to Native Americans and people from the Indian sub-continent and that these could routinely be met in the Borders too!
And the OED does not include senses relating to hair color for red man or black man.
But, it does. Sense 2a of ‘black’ is defined as ‘Having black hair or eyes’, and these are some of the citations:
1661 S. Pepys Diary 30 Apr. (1970) II. 91 Took up Mr. Hater and his wife..I find her to be a very pretty modest black woman.
1715 tr. Thomas à Kempis Christian’s Exercise i. vii. 13 The Fair, the Black, the Learned, the Unlearned, do all pass away.
1749 J. Cleland Mem. Woman of Pleasure I. 150 He might pass for what is commonly called a comely black man, with an air of distinction natural to his birth and condition.
Context makes clear that Pepys’ Mrs Hater and Fanny Hill’s ‘comely black man’ were certainly not of African ethnicity: just black-haired. Had they been, it would have been necessary to describe them as ‘African’, ‘Negro or some such term, since in the 17th and 18th centuries black did not clearly convey that. Here’s another example:
Of all the girls in our town,
The black, the fair, the red, the brown,
That dance and prance it up and down,
There’s none like Nancy Dawson
Anne Dawson became a star overnight when she danced the hornpipe in The Beggars’ Opera in 1760 (at which time the London theatre scene was certainly not swarming with black, Native American and Indian dancers!)
And sense 5a of red is ‘having hair of an auburn or ginger colour’.
But if the film is, say, an episode of Dr. Who that is set in Roman Britain, then there is absolutely no reason for not including black actors
I think there is. In any drama that deals with the irruption of something fantastic, surreal or farcical into a real milieu, that milieu must be portrayed realistically; if it is unreal or implausible, the premise of your drama fails. This accounts for the often-remarked (by historians) fact that the mises-en-scene of historical comedies are typically far more meticulously researched and created than those of serious historical dramas; their makers know that if you don’t ground farce in reality, it falls flat.