Why did the artist most likely color the hair against a black-and-white background?

To illustrate a person with limited imagination

I think it is because the artist might have had a background story behind it and that's probably why he/she colored it. Also to draw your attention to a specific aspect of the picture. Try searching up why the artist did what he/she did

Answer humanism;

∵they depict sad lonely people  ∵  figures in them have long limbs and faces, similiar in style to paintings by el greco.