Li Yuan-Chia photographs

I was at the Whitworth Gallery in Manchester yesterday and came across the Li Yuan-Chia exhibition -

His technique was to print his photographs in black and white and then hand colour them. Given that, I suspect, few would have either the skill or time to do so I was wondering if anyone had any ideas on how to do this in software?

This is actually an old way to produce photographs in colours, invented during black & white era, the time before realistic looking color prints existed. I have considered doing the same, coloring just selected parts of a photo. In Gimp you can make a new layer, set blending mode to color, brush paint on it and set opacity to taste. I guess a good Wacom-type tablet can be highly useful.

I think that the whole point of this process is to combine the image data layer with a manual, and unique layer, of brush work. The use of Gimp or other digital paint program misses that mark entirely.
Recently I have seen a number of artists using acrylic paints so that the layer distinction becomes more apparent. In the work that I have seen the ‘paint’ elements do not by necessity attempt to mirror the digital underlay.