This next photos are the example of a technique that I use to give color to old black and white pictures. I won’t lie! Takes a lot of time and patience to achieve it. But if you are “obsessed with color” (like me) and have a lot of hours to kill time, why not give it a try?? I use Gimp. I duplicate several layers with different opacity (and colors). Then I paint the “base layer” using the brush tool. In the end, I merge the layers and save the image.
If your eyes get too tired, you can always save it as a project and continue it later.