Aw, I just love old photos like those. Yes, there was no color photography in the early days so the portrait painters, who had originally feared and hated this new-fangled medium called "photography" because they thought it would drive them out of business, suddenly had a new way to make money. People looked at these "true" images of themselves and were not always gladdened by what the camera captured. Where were those rosy cheeks, those soft blue eyes??
So, oils were applied directly onto the photograph to bring back that color people missed. That's the beginning of handcoloring, or hand tinting, photos. Eventually, more chemically sound photo oils were developed that contained archival properties that didn't eat away at the paper but rather enhanced the image's durability - which is why we can still admire some of these photos today. Marshall's Photo Oils remain the best.
Handcoloring now takes on many different forms. You can tint an image digitally if you like the idea of working only on your PC and not investing the time and money to learn the classic technique. Or, you can learn the technique and experiment with all kinds of coloring mediums: photo oils, oil sticks, pencils and pastels (chalks), markers - the list goes on.
Mitica is right: Handcolor.com is an interesting site with articles and reference materials, though it's no longer the interactive site it once was. But it will point you in the right direction, and they have additional links, plus a bookstore online, and you can also purchase handcoloring materials from them (although your local arts supply store might be cheaper) if you're interested.
As you can tell from my enthusiasm, I love this technique as an additional creative outlet that goes hand in hand with B&W photography, which I also love. Once you get bitten by the bug, look out!
Have fun learning. Hope this helps!