system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: How do photo restoration artists know the supposed colors of greyscale images?

Are the colors based purely on their assumptions/imagination, or do the greyscale images retain some sort of data that tells what color on what part?
gpt
I often color old black-and-white photographs as a hobby and, most of the time, you just have to guess when you colorize an image. 

Sometimes, for famous people, you can do historical research to find out the color of their hair and eyes or even their homes' furnishings (for instance, Empress Alexandra Feodorovna had reddish gold hair and blue eyes and her mauve boudoir was all lavender with pistachio green carpets). You can also do research to get a general idea of the kinds of colors that were popular, say, in the Victorian or Edwardian eras... but that's pretty much it.