MyHeritage has a photo colorizer. It is free through 4/22 (tomorrow- hurry!), according to the FAQ. You do have to create an account on their site to use it. Normally non-subscribers can colorize up to 10 photos for free.
I decided to try it out and below are the results. A lot of the photos I tried were too small. I think they have to be at least 200×200, although it doesn’t say in the FAQ.