I get to restore quite a few images like
this one . Often the only coloured part is the background and, maybe, some detail on the clothing.
I imagine that, at that time, colourizing (by painting) was not an easy task and the less they could do, and still look coloured, the better.
Today it is a good technique which simplifies the work (les to colour) and brings more attention to the subject,
like this .
Though I don't know a specific name for this.
Rô