Eli5: How do photo restoration artists know the supposed colors of greyscale images?

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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?

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Different shades of grey align with different colors. So a really dark shade would likely be a dark blue/red/purple. Then it comes down to knowing the fashion trends and what colors were used at the time,in that region, and the economic class of the person. They can also consult written accounts for colors of things like military uniforms.

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.

It is based on assumption. The grayscale image only contain the brightness of an object but not its hue or saturation. The artist will therefore have to make up these by themselves. Often you can research a bit about the objects to find their hue. For example peolpe have the same hue skin color, militaries have the same hue uniforms, etc. The saturation on the other hand is more or less guesswork. And I have seen restaured photos with the completely wrong hue for objects.

>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?

More of the former and less of the latter.

I colorize photos as a hobby and have posted a few on Reddit. When picking a color it’s a process. For me it’s all about context and light levels. Light levels are one of the easiest problems that will make a colorized photo look weird. A bright color being added to a part of the photo that was originally dark or vice versa will just make it look abnormal.

My process for picking colors tends to go like this

1. Research – find color photos or descriptions of the subject to use as a template. I’ve spent many hours looking up tiny details like military service medals.
2. Balance – If research doesn’t work, find colors that match the overall look of the photo
3. Guess – If neither 1 nor 2 work, just throw a color on there and hope for the best (make sure your light levels match up)

I had one photo of a building from the 1930s I worked on for about a month. My research told me it was a brick building from the 1800s to the current day. I kept trying to add red brick colors to it and it always looked weird and too bright. After a month or so, I noticed the department store that occupied the building from the 1920s to 1940s always painted their other buildings white. I had been trying to add a dark red brick color to a white building and it just looked weird. I ended up scrapping my work on that photo and moved on because all the buildings on the street were some form of white or grey.