DeOldify is a set of Python/Jupyter/Machine Learning tools to add color and generally try to rescue aging photographs. Fully Open Source and requiring fairly heavy-weight hardware I thing that it is one to keep an eye on.
Some examples from the project link:
Maria Anderson as the Fairy Fleur de farine and Lyubov Rabtsova as her page in the ballet “Sleeping Beauty” at the Imperial Theater, St. Petersburg, Russia, 1890.
Edinburgh from the sky in the 1920s
There is even a Colab version that lets you try it out without installing any software (you will need a Google Drive account): https://colab.research.google.com/github/jantic/DeOldify/blob/master/DeOldify_colab.ipynb