@MarkL, also look for the book "Painted Ladies" by James Wedge, also "Obsessions" & "The Dark Summer" by Bob Carlos Clarke.
There are many different techniques,
there are some examples here I made during an impromptu lecture in the mid 1980s. I favoured using toners and dyes, the technique Bob Carlos Clarke was using around the same time. We both also used colour couplers, essentially you bleach a B&W image in a re-halogenating bleac,h, washing, then re-expose and re-developer in a colour developer adding a few drops of the appropriate colour coupler (or mix). There were commercial kits,
1948 Johnson Colourform, and later a Tetenal kit which was what Bob Carlos Clarke used with Agfa papers for many of the images in "Dark Summer".
View attachment 390817
I've posted this image before, I'd guess made early to mid1961, the hand colouring is remarkably accurate. While the photographer made this image, and an image of my youngest sister on her own to match the pair at the top of the image, I now have all three, his wife was making copious notes and adding water colour reminders. She did the hand colouring and was remarkably skilled.
It's important to realise there are quite different approaches in terms of technique, and also even more ways of interpretation with hand colouring. I mentioned James Wedge his hand coloured work was largely advertising based and superb, Bob Carlos Clarke was a lingerie and advertising photographer who's work bordered into the art world, he was also one of the earliest art photographers making Lith prints in the 1970s.
Ian