Hand-coloured Black & White photographs were popular in the late nineteenth and early twentieth centuries, although the process dates back to the earliest days of photography. The purpose was to add realism to the prints.
I was told that the actual process of applying colour to a black and white print is not that difficult, but that is not my experience! The artistry, as always, comes in the skill and choices made by the colourist. The artist may colour the whole print or just a portion of it, using oil paints and/or colour pencils and the final results can be subtle or dramatic.
The colourists all but disappeared with the advent of colour photography and later the introduction of Photoshop sadly brought the traditional art form and profession to a virtual end.