Thought this would be solved by looking in the Roy Bacon book, but no. '50,,51,52 are describes as painted parts "either Devon red or black" and all the pictures are black and white

.
Even more annoyingly in the black and white photos some badges look grey and others black with no particular pattern.
Also bikes "built in '51" for the UK market have painted tanks due to the nickle shortage. I'm assuming yours is export and chromed?
You could put a red one on one side and a black on the other and see how people react, then make both the colour that the majority think is right. (if they even notice

).