I am reading a news article(https://www.bbc.co.uk/news/in-pictures-55619618) about B&W photographs from the civil rights movement converted to colour. I am curious how is the colour choosen for the dresses, skintones, skies, background etc? Is it related to guess work or is there some standard for matching grey pixels to colour pixels? I have seen B&W movies converted to colour and been mesmerised, for example this one: the original - https://www.youtube.com/watch?v=grytrusvVag and the coloured: https://www.youtube.com/watch?v=dOe-B9dJcIs